In the crawl setting, one can specify to index a site in the www wild. So, it stands to reason that if it needs to index that site, that it would have to ping that site for info, which would require bandwidth.
I don't know this for a fact, but it should be easy enough to test. You can create a test crawl to a site and manually run it, whilst monitoring the bandwidth.
So, it is just a guess, but my logic makes sense to me... I think.
Steve Clark, MCTS | Twin-Soft.com