Colleges ask students to submit the things they'd like to receive in an application. Everyone lays out their requirements somewhere. On the UVa website, we have a little chart that lists all the different components of an application with deadlines for the different kinds of applicants (btw, if you use an old version of IE, the chart comes out a little wonky...I'm not sure why). We also have a page that explains the different supplements that some students might opt to submit if they are interested in the arts or architecture. If you aren't submitting an art supplement or architecture portfolio, just focus on filling out the Common App.
Over the years, more and more people have taken to submitting extra items that don't fit into the parameters of the art and architecture supplements. I think the assumption is that either the Common App, long though it may be, doesn't provide enough information or that the exceptional student must go to extraordinary lengths to convey their awesomeness.
By the way, did you know what we have a special place for anything that comes in that doesn't fit into the application? It's called the "Not Art Supplements" bin. We don't look at the things that go into that bin. It'd probably be a lot of fun to go through those bins, but with 29,000 applications to review in just a few months, we don't really have time for that.
The good news is that if you are a busy high school student with many commitments on your plate, you don't have to spend time crafting elaborate application documents. The required credentials are exactly what we need for our review.