Prayer is a unique training and developmental program which, if well and devotedly performed, can achieve for a Muslim many valuable spiritual, physical, and ethical gains.
The importance of the prayer in Islam cannot be understated. It is the first pillar of Islam that the Prophet, may God praise him, mentioned after mentioning the Testimony of Faith, by which one becomes a Muslim. It was made obligatory upon all the prophets and for all peoples. God has declared its obligatory status under majestic circumstances. For example, when God spoke directly to Moses, He said,
“And I have chosen you, so listen to that which is inspired to you. Verily, I am God! There is none worthy of worship but I, so worship Me and offer prayer perfectly for My remembrance.” (Quran 20:13-14)
Similarly, the prayers were made obligatory upon the Prophet Muhammad during his ascension to heaven. Furthermore, when God praises the believers, such as in the beginning of the chapter entitled ‘al-Muminoon’ (the Believers), one of the first descriptions He states is their adherence to the prayers.
Once a man asked the Prophet about the most virtuous deed. The Prophet stated that the most virtuous deed is the prayer. The man asked again and again and for the first three times, the Prophet answered, “The prayer,” then on the fourth occasion he stated, “Jihad in the way of God.” 
The importance of prayer is demonstrated in many of the Prophet’s statements. For example, the Prophet said,
“The first matter that the slave will be brought to account for on the Day of Judgment is the prayer. If it is sound, then the rest of his deeds will be sound. And if it is incomplete, then the rest of his deeds will be incomplete.” 
The importance of the prayers lies in the fact that no matter what actions one performs in his life, the most important aspect is one’s relationship to God, that is, one’s faith (imaan), God-consciousness (taqwa), sincerity (ikhlas) and worship of God (ibaadah). This relationship with God is both demonstrated and put into practice, as well as improved and increased, by the prayer. Therefore, if the prayers are sound and proper, the rest of the deeds will be sound and proper; and if the prayers are not sound and proper, then the rest of the deeds will not be sound and proper, as the Prophet himself stated.
In reality, the prayer is performed properly – with true remembrance of God and turning to Him for forgiveness – it will have a lasting effect on the person. After he finishes the prayer, his heart will be filled with the remembrance of God. He will be fearful as well as hopeful of God. After that experience, he will not want to move from that lofty position to one wherein he disobeys God. God has mentioned this aspect of the prayer when He has said,
“Verily, the prayer keeps one from the great sins and evil deeds” (Quran 29:45)
Nadwi has described this effect in the following eloquent way,
Its aim is to generate within the subliminal self of man such spiritual power, light of faith and awareness of God as can enable him to strive successfully against all kinds of evils and temptations and remain steadfast at times of trial and adversity and protect himself against the weakness of the flesh and the mischief of immoderate appetites. 
The overall affect that the properly performed prayers should have upon humans is described in other verses in the Quran:
“Verily, man was created impatient, irritable when evil touches him and ungenerous when good touches him. Except for those devoted to prayer those who remain constant in their prayers…” (Quran 70:19-23)
As for the Hereafter, God’s forgiveness and pleasure is closely related to the prayers. The Messenger of God said,
“God has obligated five prayers. Whoever excellently performs their ablutions, prays them in their proper times, completes their bows, prostrations and khushu, has a promise from God that He will forgive him. And whoever does not do that has no promise from God. He may either forgive him or punish him.”
The prayers are a type of purification for a human being. He turns and meets with his Lord five times a day. As alluded to above, this repeated standing in front of God should keep the person from doing sinful acts during the day. Furthermore, it should also be a time of remorse and repentance, such that he earnestly asks God for forgiveness for those sins that he committed. In addition, the prayer in itself is a good deed that wipes away some of the evil deeds that he performed. These points can be noted in the following hadith of the Prophet, may God praise him:
“If a person had a stream outside his door and he bathed in it five times a day, do you think he would have any filth left on him?” The people said, “No filth would remain on him whatsoever.” The Prophet then said, “That is like the five daily prayers: God wipes away the sins by them.” (Saheeh Al-Bukhari, Saheeh Muslim)
In another hadith, the Prophet said,
“The five daily prayers and the Friday Prayer until the Friday prayer are expiation for what is between them.” (Saheeh Muslim).