One of the reasons why the book of Psalms was written is to give us a Biblical template to express grief, discouragement, complaint, anger and the whole gamut of human emotions. It’s as if God is saying, “Go ahead and tell me your predicament. Your feelings don’t shock me at all. I’m listening and I care.”
Other uses of the book of Psalms include the following:
Prophetic- many Psalms are prophetic in nature. Many descriptions of the Messiah come from the Psalms, like the one where He was to be pierced on the side and that he will not see corruption, i.e., He will be resurrected.
Theology- Many of the grandest ideas about the infinitudes of God come from the Psalms. Psalm139 is a great example.
Worship- The Psalms gave us ideas how to approach God. Without the Psalms, we would have no idea how personal worship works.
Psychology- Every one who reads the Psalms is bound to notice how the outworking of the human mind is carefully explored in this book.
Poetry- the rhythm and linguistic beauty of the Psalms are rare, considering that the book was compiled thousands of years ago.