This is a tough thread to get focused in!
I think it would be ideal to start your own thread with the question as the OP, so that moderation can keep such a thread on track. There is a wealth of information on the PB concerning this, in case you haven't used the search feature for that. I can try to suggest a bit of reading material when I have time, and maybe others on the thread can, as well. But the beginnings of the answer can be found in things like the fact that the Psalms were what the church in the OT and NT sang and used for worship; our Lord sang them (and inspired them); the name of the psalter in Hebrew is "Book of Praises," showing that this is the purpose of the book; those are just a few thoughts.