Why are there viruses?

Terrorism?
Evolution?
Symantec?
Coolness?

I think they exist because they can and people will do whatever they can to get their fifteen minutes of fame.

Are they actually a good thing? One reason to think so is that they have forced developers to harden their systems against attack. The aliens in the movie "Independence Day" obviously had never heard of viruses (much like HGWells' aliens had not prepared for Earth's microbes.)

Before we do much interplanetary exploration we are going to need some really broadband bacteria and virus protection. Do you remember the quarantine imposed on the Apollo 11 astronauts when they returned from the moon in 1969.