Why do they do this?

There are as many reasons as there are trolls. Many trolls are just
practical jokers attempting to amuse themselves and their audience.
Some have genuine animosity toward a particular group or individual
that they believe has done them wrong. Some are just loathsome,
contemptible hooligans bent on disrupting the ordinarily peaceful
exchange of useful information for their own perverted pleasure.

Usenet provides a medium to create and express oneself, and gives the
added dimension of anonymity, allowing the troll to create and express
emotions, thoughts and ideas that he/she might not ordinarily express.
For many, Usenet is the first and only place their creative works will
ever be "published". Trolling, for many, is a creative endeavor, and
art form if you will, misunderstood by many, appreciated by a few, but
valid to the writer none-the-less.

JDA~today