These bots are usually scrapers, data miners or email harvesters.
You can find some related articles on this blog: http://www.spam-whackers.com/blog/
Other than that they are eating up your bandwidth so cost you money 
Beside the bad bot IP’s list in my previous post my first todo is deny every traffic from *amazonaws.com when I set up a new domain. Although it is a service from amazon, it is mostly used to host bots and hacker bots. See more here: http://forums.oscommerce.com/lofiversion/index.php?t303607.html