There are many reasons why.
Lots of times someone will bump it asking the OP for an answer, feedback or if the issue was resolved and the OP isn't around any longer.
Lots of times people bump just to say "Nice thread"
Lots of times People bump with add on questions or non-relevant info.
There are many reasons. Bottom line though is that the owner and staff would prefer you start a new thread with the info you have and go from there.
This is very true.
So many times in the pics and vids section I've seen people randomly bump threads for no reason just to say something like "nice pics". To me that's pointless. If there is thread on an issue that wasn't resolved and a member found a solution for it that would benefit others, I don't see why bumping it would be a bad idea, especially if there was other useful info in said thread.