Translation... Think of the entire forum as a big notebook. Everyone posting in the notebook is writing on the lines of each page. The more posts, the more lines written on, the bigger book you need.
NOW... let's assume that every time someone writes something on a page, he or she also draws a big picture next to what is written... the pictures would be Avatars.
NOW... Think how much faster you'd need to add pages to your book if everyone drew a picture before writing what they write.
Make more sense now?
(I don't know how much space avatars take up... I'm merely translating)