From Institutes III.xx.2
(1) Prayer brings us the blessings that God has promised.
“It is by the benefit of prayer that we reach those riches laid up for us by the Heavenly Father.”
(2) There is safety and security in calling upon His name (Joel 2:32; cf. Prov. 18:10).
Joel 2:32: “And everyone who calls on the name of the Lord will be saved; for on Mount Zion and in Jerusalem there will be deliverance, as the Lord has said, even among the survivors whom the Lord calls.”
Proverbs 18:10: “The name of the Lord is a fortified tower; the righteous run to it and are safe.”
(3) Prayer invokes the providence of God.
“through which he watches over and guards our affairs”
The Heidelberg Catechism, Questions 27 and 28 are informative here.
(4) Prayer invokes God’s power.
“through which he sustains us, weak as we are and well-nigh overcome”
(5) Prayer invokes God’s goodness.
“through which he receives us, miserably burdened with sins, unto grace”
(6) “…it is by prayer that we call him to reveal himself as wholly present to us.”
“Hence comes an extraordinary peace and repose to our consciences. For having disclosed to the Lord the necessity that was pressing upon us, we even rest fully in the thought that none of our ills is hid from him who, we are convinced, has both the will and the power to take the best care of us.”