Google bot, as well as all the others search you website for key words and phrases and indexes your site on it's search engine. By denying access to your website for the bot could well lose you visitors, the more active and more activity on your site, there content you will have and so bots can have more things to 'tag' you as under their search engines.
It is also pretty much the same with media bots, they look for images and sound items whereas normal bots just look at words etc...