Why are Muslims expected to prayer 5 times a day?

In a Nutshell Salah (prayer) is the second of Islam's five pillars. It comprises of collective and individual prayer obligations for every mukalaf (a Muslim of full age and liable for religious duties). Besides, it has countless benefits for Muslims, whether in this world or the hereafter e.g. unity of Muslim, building a connection with Allah, purity of body and soul, tranquillity of the mind, kafarah (expiation) for sins and remembrance of Allah as well as forbidding wickedness. Background The performance of the five daily prayers is obligatory for every mukalaf (a Muslim of full age and liable for religious duties). Besides, offering these five daily prayers has great benefits for Muslims, whether in this world or the hereafter. Although the benefits of five daily prayers are countless, we will try to reveal some of the unlimited secrets of this great worship. Showing Submission to Allah When a Muslim offers the five-daily prayer, he shows his submission and surrenders to his Lord. Allah says:  قُلْ إِنَّ صَلَاتِي وَنُسُكِي وَمَحْيَايَ وَمَمَاتِي لِلَّهِ رَبِّ الْعَالَمِينَ Say, "Indeed, my prayer, my rites of sacrifice, my living and my dying are for Allah, Lord of the worlds. (Qur'an 6:162) Unity and Brotherhood of Muslims The performance of congregational prayers in the mosques behind an imam is symbolic and mirrors the structure of an Islamic polity - namely a leader, the imam or khalif, with the jama'ah behind him in orderly obedience and support. Praying congregational prayers in their fixed times emphasises and installs this in all Muslims, promotes the unity of Muslims, seeking the same qiblah (direction of prayer). It also shows that all Muslims should be equal in the sight of Allah, standing at the same row, no difference between rich and poor. This promotes the concept of brotherhood among Muslims and consolidates their bonds with each other. Building a Connection with Allah (swt) Salah is the connection between Allah (swt) and His servants. The Muslim through his salah communicates directly with his Lord without the need of any intermediary. عَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ : أَقْرَبُ مَا يَكُونُ الْعَبْدُ مِنْ رَبِّهِ وَهُوَ سَاجِدٌ ، فَأَكْثِرُوا الدُّعَاءَ - مسلم 4/245 Abu Huraira (ra) narrated: The Messenger of Allah (saw) said: The nearest a servant comes to his Lord is when he is prostrating himself, so make supplication (in this state). (Sahih al-Muslim 4:245) Purity of Body and Soul Muslims must be in a ritually pure state to pray the five daily salah through ablutions (wudu) or ghusl (washing all the body) the cases of major impure states. Wudu, for example, involves washing the hands, mouth, nose, face, hair, ears, arms, and ankles. This ensures the removal of smells and dirt and ensures the purification of the body for the whole day. As wudu achieves the physical purity of the body, salah achieves the purity of the soul. It is a ritual purification in which minor sins are washed away, as mentioned in the hadith: عَنْ أَبِي هُرَيْرَةَ، أَنَّهُ سَمِعَ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ أَرَأَيْتُمْ لَوْ أَنَّ نَهَرًا بِبَابِ أَحَدِكُمْ، يَغْتَسِلُ فِيهِ كُلَّ يَوْمٍ خَمْسًا، مَا تَقُولُ ذَلِكَ يُبْقِي مِنْ دَرَنِهِ ‏"‏‏.‏ قَالُوا لاَ يُبْقِي مِنْ دَرَنِهِ شَيْئًا‏.‏ قَالَ ‏"‏ فَذَلِكَ مِثْلُ الصَّلَوَاتِ الْخَمْسِ، يَمْحُو اللَّهُ بِهَا الْخَطَايَا ‏"- البخاري 9/7، ومسلم 5/358 Abu Huraira: (ra) narrated: I heard Allah's Messenger (saw) saying, "If there was a river at the door of anyone of you and he took a bath in it five times a day would you notice any dirt on him?" They said, "Not a trace of dirt would be left." The Prophet (saw) added, "That is the example of the five prayers with which Allah blots out (annuls) evil deeds." (Sahih al-Bukhari 9:7; Muslim 5:358) Tranquillity and Serenity of the Mind By offering salah in the right manner, the Muslim's heart should become calm and tranquil as Allah mentioned in the following verse: الَّذِينَ آمَنُوا وَتَطْمَئِنُّ قُلُوبُهُم بِذِكْرِ اللَّهِ ۗ أَلَا بِذِكْرِ اللَّهِ تَطْمَئِنُّ الْقُلُوبُ -  الرعد 13/28 Those who have believed and whose hearts are assured by the remembrance of Allah. Unquestionably, by the remembrance of Allah hearts are assured." (Qur'an  13:28) Kafarah (expiation) for Sins Offering prayers give Muslims the opportunity to expiate sins and ask Allah for forgiveness. The Prophet (saw) demonstrated the significance of the five-daily prayer in erasing one's sins in many hadiths: عن حُمْرَانَ بْنَ أَبَانَ أن رَسُولَ اللَّهِ صلى الله عليه وسلم قال قَالَ ‏"‏ مَا مِنْ مُسْلِمٍ يَتَطَهَّرُ فَيُتِمُّ الطُّهُورَ الَّذِي كَتَبَ اللَّهُ عَلَيْهِ فَيُصَلِّي هَذِهِ الصَّلَوَاتِ الْخَمْسَ إِلاَّ كَانَتْ كَفَّارَاتٍ لِمَا بَيْنَهَا‏ ‏ - مسلم 2/13 Humran ibn Aban (ra) narrated  "I used to fetch water for Uthman for his purification. Never was there a day that he did not take a bath with a small quantity of water. And Uthman said: The Messenger of Allah (saw) said: A Muslim who purifies (himself) and completes purification as enjoined upon him by Allah and then offers the prayers, that will be expiation (of his sins he committed) between these (prayers)." (Sahih al-Muslim 2:13) عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَال:‏ الصَّلَوَاتُ الْخَمْسُ وَالْجُمُعَةُ إِلَى الْجُمُعَةِ كَفَّارَاتٌ لِمَا بَيْنَهُنّ - مسلم 2/18 Abu Huraira (ra) narrated that the Messenger of Allah (saw) said "The five (daily) prayers and Friday prayer to (the next) Friday prayer are expiations (for the sins committed in the intervals) between them." (Sahih al-Muslim 2:18) عن عثمان بن عفان رَضِيَ اللَّهُ عَنهُ قال سمعت رَسُول اللَّهِ صلى الله عليه وسلم يقول: ما من امرئ مسلم تحضر صلاة مكتوبة فيحسن وضوءها وخشوعها وركوعها إلا كانت كفارة لما قبلها من الذنوب ما لم تؤت كبيرة؛ وذلك الدهر كله -  مسلم 9/56 Uthman ibn Affan (ra) narrated he heard the Messenger of Allah (saw) saying, "When the time for a prescribed Salah is due and a Muslim performs its Wudu and its acts of bowing and prostration properly, this Salah will be an expiation for his past sins, so long as he does not commit major sins, and this applies for ever." (Sahih al-Muslim 9:56) Remembrance of Allah Salah is also a remembrance of Allah. Allah says: إِنَّنِي أَنَا اللَّهُ لَا إِلَٰهَ إِلَّا أَنَا فَاعْبُدْنِي وَأَقِمِ الصَّلَاةَ لِذِكْرِي - طه 14/20 Indeed, I am Allah . There is no deity except Me, so worship Me and establish prayer for My remembrance. (Qur'an 14:20) The remembrance of Allah disciplines the mind, controls inordinate desires and reins the rebellious soul. Forbidding Wickedness and Evil Deeds The proper and sincere salah deters the Muslim from committing evil deeds. Allah says. اتْلُ مَا أُوحِيَ إِلَيْكَ مِنَ الْكِتَابِ وَأَقِمِ الصَّلَاةَ ۖ إِنَّ الصَّلَاةَ تَنْهَىٰ عَنِ الْفَحْشَاءِ وَالْمُنكَرِ ۗ وَلَذِكْرُ اللَّهِ أَكْبَرُ ۗ وَاللَّهُ يَعْلَمُ مَا تَصْنَعُونَ - العنكبوت 29/45  Recite what has been revealed to you of the Book and establish prayer. Indeed, (genuine) prayer should deter (one) from indecency and wickedness. The remembrance of Allah is (an) even greater (deterrent). And Allah (fully) knows what you (all) do. (Qur'an 29:45) Conclusion There are countless benefits, whether in this world or the hereafter, one can find from these daily prayers, such as the unity of Muslim, building a connection with Allah, purity of body and soul, tranquillity of the mind, kafarah (expiation) for sins and remembrance of Allah as well as forbidding wickedness.