For me it was always a way to say what I couldn't to anyone else. I've not written a poem in many years now - which is a good thing - I wrote 500+ poems from the view of an angsty, suicidal teenager. Something I am not now. Not angsty, nor sucidal, errr...nor a teenager (man, now I feel old).
It was some way of getting out what I needed. They were fairly dark, but accurately portrayed what I was feeling - about life, and about myself, at the time.
Occasionally it was frustration at hatred I encountered - not necessarily at me - but at things I had an inkling I would grow to understand better and felt offended that someone would be so bigoted and hateful about a group of people (and yes, I grew to understand MUCH better...)
It probably saved my life more than once. I would write instead of doing. Though, I've looked back at some of them - and i know it's egotistical but...some of them were bloody good!