A lot of the time I use it if it was a truly useful post - info that I didn't know before, or something explained well.
Sometimes I just use it when someone has already said exactly what I was going to say, but there's really no reason for me to repeat it. So then I feel like the "thanks" lets other people know that, yep, this is good info here, even if there are no other posts directly saying that it is.
Juno CGC: 3/31/09 "Reverse" Brindle, docked & floppy
Lucy: 2/1/06 Bichon/Poodle Mix
Happy: 1/2001, Rescued Border Collie Mix
RIP Molly, January 1997-April 8, 2009