Static cling
Web pages can have a lot of moving parts. The average site is much more than just plain HTML. There are Cascading Style Sheets (CSS) to define the look and feel, image files, JavaScript, and other elements to control and define the appearance and behavior of your page. In order to incorporate all these things into a Visualforce page, you'll need a place to store them. You can host them on an external web server, but that makes maintaining things more cumbersome. Why not host these files in the same place where you host your Visualforce pages, on the Salesforce1 Platform?
Static resources are files uploaded to the Salesforce1 Platform that can be used directly from Visualforce. Unlike Visualforce and Apex, they're uploaded and managed by system administrators from the setup console. The average end users can't access them directly, so they're typically changed less often than other files. The great news is that you don't need to write code to query for them, so they don't count...