I can only answer for myself, others may have different reasons.
Way back when I joined this site a couple of years ago, I was single and looking for a relationship. During my time on this site, I came to love reading and sometimes posting in the forums.
Then, last October I met someone (not from this site) who I did want to have a committed relationship with. For awhile I simply changed my profile to have a picture of the two of us and to read something to the effect of "I am no longer looking and only keep this profile active because I love the forums on this site".
Sadly, that didn't stop people from winking and emailing me, and assuming I was looking to cheat, etc. So, I simply hid my profile. Allows mw to participate in the forums, but is pretty clear about the fact I am not looking for anything more than I have.