What does the AdvAgg bundler do?
The short answer is to take advantage of the browser cache. It can also be used to take advantage of the number of parallel connections that the browser can make. 

Example:
Front page has a.css and b.css; node 1 has a.css, b.css and c.css. If using one aggregate then the user going from the front page to node 1 would have to download all of the CSS again. By splitting up the aggregate in a smart manner the user will only need to download c.css