Well, I do it out of kindness to the person, acknowledging that I've seen their question, sympathize, and wish I could help. Sometimes I'd also like to know the answer myself, and it's an easy way to mark.
I guess you could look at it another way: would you want a hundred views and no acknowledgement at all? Would you feel like you've asked a dumb question or we were unfriendly if no one said hello? I would. Which is why I try not to do that.