I think the reason posted by Gymrat76 is the most valid.
+1 before lock is used to alert the Mods of a potential problem thread as it will bubble the thread to the top (or near the top) of searches. Sometimes, due to the number of posts in the Forums, we do miss out potential problem threads and having someone bubble it up brings it to our attention.
Of course, there are other ways to alert us:-
a) send a PM to the Mods/Admin
b) Click on the Report Post button to send us an email
Whatever works, I guess.