As Mike said, spiders add pages to their database. They also attach key words to them, based on the content (text) they find. Spiders (generally) only see text. Without spiders, sites wouldn't get indexed by search engines like Google. Not a big deal to you all, but a very big deal to Martin and I.
Bots, on the other hand, generally scrap personal information like email addresses off web pages, so they can be sold to marketing companies. This is one of the ways you get spam.