When the troll can't respond with actual arguments, he will more often than not respond with insults and personal attacks. By insulting the "opponent", the troll hopes to divert attention away from his lack of argument and to start an argument over the insult instead.
It is also often used as a one-liner as a response to a post made by someone the troll dislikes (see Thread hijacking) when that person is having a conversation with someone else. The troll will reply to the post, usually snip most of the post away and then add an insult to one part of the post.