It is like a picture, in which you see yourself portrayed, and seeing, may understand and consequently form yourself upon the pattern given.
Elsewhere in the Bible you read only that the Law commands this or that to be done, you listen to the Prophets to learn about the Saviour’s coming, or you turn to the historical books to learn the doings of the kings and holy men.
But in the Psalter, besides all these things, you learn about yourself.
You find depicted in it all the movements of your soul, all its changes, its ups and downs, its failures and recoveries.
Moreover, whatever your particular need or trouble, from this same book you can select a form of words to fit it, so that you do not merely hear and then pass on, but learn the way to remedy your ill.
Prohibitions of evil-doing are plentiful in Scripture, but only the Psalter tells you how to obey these orders and abstain from sin.
Repentance, for example, is enjoined repeatedly; but to repent means to leave off sinning, and it is the Psalms that show you how to set about repenting and with what words your penitence may be expressed.
Again, Saint Paul says, Tribulation worketh endurance, and endurance experience, and experience hope, and hope maketh not ashamed (Rom 5:3, 5); but it is in the Psalms that we find written and described how afflictions should be borne, and what the afflicted ought to say, both at the time and when his troubles cease.
The whole process of his testing is set forth in them and we are shown exactly with what words to voice our hope in God.
Or take the commandment, In everything give thanks (1 Thess 5:18). The Psalms not only exhort us to be thankful, they also provide us with fitting words to say.
We are told, too, by other writers that all who would live godly in Christ must suffer persecution (2 Tim 3:12); and here again the Psalms supply words with which both those who flee persecution and those who suffer under it may suitably address themselves to God, and it does the same for those who have been rescued from it.
We are bidden elsewhere in the Bible also to bless the Lord and to acknowledge Him: here in the Psalms we are shown the way to do it, and with what sort of words His majesty may meetly be confessed.
In fact, under all the circumstances of life, we shall find that these divine songs suit ourselves and meet our own souls’ need at every turn.
Athanasius of Alexandria (c.293-373): Letter to Marcellinus in Athanasius: The Life Of Antony And The Letter To Marcellinus, translated by Robert C. Gregg; Paulist Press, New York; pp. 101-129; 1980 @ Athanasius.com