Badges are designed as few 'gaming' devices on the site. The first that comes to mind is flair!
They show the community and other users accomplishments you've made in the community. While some badges are easier to get than others none are really trivial - It's not easy to ask a question that gets 'untouched' for a tumbleweed period of time, on the same note necromancer is to highlight those users taking care of tumbleweeds.
Brian has 37 pieces of flair.
Second, it's a gaming device. StackExchange is constantly improving the network and will occasionally add badges to help keep things interesting. Ideally there here to help users have objectives other than just answer questions for what basically amounts to no compensation (Outside of reputation and unicorn bucks)
So you can view each badge, what it means, and who has gotten it on the Badges page each has it's own meaning and level of difficulty to achieve (Bronze is 'easiest', Silver is 'relatively hard', and Gold is 'got to really try hard to get this').