Catharsis.
There may be other factors involved,but my prime mover for writing songs is to get whatever is on my mind..off my mind via the medium of musical composition. I don't think I would go insane if I couldn't write,but I'd certainly be more stressed and much less content.
That feeling which comes when you finally stop fiddling about and let the song go,is cathartic. It's a release,a lancing of a troublesome boil and brings a feeling of fulfilment. The DAW is my shrink. (There's a song in that...)
Only temporary,but hey,I'll take it.
I like some songs not to be at all autobiographical,but if folk get that out if it,well I let it fly anyway.
I enjoy vicariously soaking up the experiences of others and suffering on their behalf...sometimes!
Use every avenue available to catch a song from the ether.