Some do it to break down the post and reply individually to each sentence/statement. I've done this myself. It's mainly used during an arguement in order to carefully counter all their points, so you quote them and reply. It involves a lot of edits to the quotes tags tho. Another reason is to point out something funny or something that stood out from the rest. Or when they're trying to enforce what was said in the quote. And the last reason is copy/paste. But most just delete the quote tags and the reply itself after they've gotten the copy into their clipboard. Then there's people like this idiot who quotes the whole post for no apparent reason.