Proper prayer is one said with complete intent, and functions as the core of a Jew’s day. Prayer is spiritual sustenance for a person’s soul, sustaining it until the next prayer.

Thoughtful prayer, said with concentration, is the focus of the day:

The prayer of a pious person is not one of custom or habit, like the song of a starling or parrot. Rather, every word is accompanied by thought and concentration. In this way, for the pious one, the time of prayer forms the core and purpose of his time. Other times are like paths that lead to this time [of prayer], for he looks forward to its coming. Through such prayer, he becomes similar to the spiritual entities and distances himself from animalistic tendencies.

Prayer is sustenance for the soul and sustains it from prayer to prayer:

The value of prayer for the soul is like that of food for the body. For a person’s prayer is good for his soul just as food benefits his body. The blessing that results from each prayer rests upon him until the time of the prayer that follows, just as the energy from the meal he ate keeps him sustained until the evening meal…. During prayer a person purifies his soul from all that it went through in the interim [since the last prayer], and prepares it for the future. (Rav Yehuda HaLevi, Kuzari 3:5)

Prayer has two central goals: First, God, who is good, wants what is good for us. He therefore gave us commandments so that, through fulfilling them, we can merit all good. He also tells us that the way to receive our needs is by praying to Him and requesting them, and this itself is also a commandment. In addition, prayer strengthens our consciousness and belief that the Holy One, blessed be He, is responsive to our needs and watches over us, and that He is omnipotent and nothing is beyond His ability.

[One] of the reasons for this commandment is what I have previously mentioned many times, that good things and blessings will be granted to people according to their actions, good-heartedness, and proper thoughts. The Master of all who created them wants what is good for them, and leads them and brings them success through His precious commandments that will bring them merit. He provided for them and informed them how to attain all of their positive desires – that is, through requesting of Him, may He be blessed, the One who has the ability and wherewithal to give them anything they lack. For He will respond from Heaven to anyone who calls to Him truthfully. Besides informing them of this [divine] quality, He commanded them to use it and to always request all of their needs and what they want. In addition to attaining what we want, through arousing our spirits and totally focusing our thoughts on His being the good Master who does good to us, we also merit that His eyes are open, [watching over] us everywhere we go. All the time, at every moment, He hears when we cry out to Him; “The Guardian of Israel neither slumbers nor sleeps” (Psalms 121:4). [Prayer also brings us merit through] believing in His kingship and His unquestionable and unlimited ability, that there is nothing that holds Him back or impedes Him from anything He wants. (Sefer HaĤinnukh 433)

Prayer involves the needs of the person praying, but it also includes an element of service of God. This is because prayer is aimed at strengthening our recognition of our complete dependence on the Creator.

Prayer is God’s will:

“The prayer of the upright is His gratification [retzono]” (Proverbs 15:8) – it is God’s will, may He be blessed. Even though prayer, where a person makes requests of God, may He be blessed, is for his own benefit, nevertheless, such prayer is the service of the upright. This refers to prayer, which is His will.

The service of prayer is to deepen belief in God:

Do not ask: How is it possible to say that prayer is service of God, may He be blessed? Do we not pray for the sake of reward? If so, why is prayer referred to as “service”? We have already explained that the entire concept of service indicates that everything belongs to God, may He be blessed…. The same is true for prayer. When a person prays before God like a servant appealing to his master for his requested needs, this prayer indicates that man needs God. If it is so that man is dependent on God, may He be blessed, he is His – for one who is dependent on another is his, as will be explained later. Therefore prayer is completely [considered] service, for it indicates that a person is dependent on God, may He be blessed, and is therefore His. (Maharal, Netivot Olam, Netiv HaAvoda 1)

Further reading: For more on divine providence over human behavior, see p. 173.

Every sickness has its own specific cure, but prayer works for everything. Prayer is effective for a sick person praying to get well, and also for someone sentenced to death who wants to be saved. It is a cure with no negative side effects.

The special quality of prayer itself is that it is effective for all things. For we find that prayer is effective for healing the sick, as it did for Hezekiah: “I have heard your prayer…behold, I am healing you; on the third day you will go up to the House of the Lord” (II Kings 20:5). It is also effective to save from death. For when Israel sinned through the Golden Calf, Moses was told, “Let Me, and I will destroy them” (Deuteronomy 9:14), and they were later saved through Moses’ prayer…. If so, prayer is like an all-purpose balm that is effective for all sicknesses and all poisons, as opposed to other types of antidotes, which are effective only for specific sicknesses. (Rav Yosef Albo, Sefer HaIkkarim, chap. 20)