I can think of a few of reasons:
1.) research... use advice here and apply it to my own relationship where I see fit
2.) ego trip... nothing more satisfying than helping others and feeling like you've actually helped
3.) social atmosphere... I genuinely enjoy the personalities of many of the 'regulars' who post here
4.) passes the time... sometimes it helps to lose yourself to something other than your own troubles... this forum helps in that regard.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen