• It keeps my room warm in the winter. I live in a garage and don't use gas for heating. A watt of power dissipated by a computer is roughly the same as a watt of power dissipated by an electric heater, except that the computer can search for aliens simultaneously!
  
• It's an excuse to have a lot of computers with diverse hardware and software configurations. They're diverse because they're junk.
  
• I think that SETI searches for the answer to one of the biggest questions humanity can have.
  

I think, given my current understanding of the universe, the existence of extraterrestrial life somewhere sometime is almost a certainty. Whether or not it's close enough to us in time and space for us to make contact could be a very slim chance however. To increase the odds in the favour of contact we have to be listening and searching.