I use it for the promos I run at Double-Cross Lit for the books that are not exclusive to Amazon. I also set the affiliate codes for those sites that I can. This way, I can see how many people clicked over from a particular book, and if they go to Apple, or Smashwords or Amazon to buy, I get a small commission.
Also, if they go to Amazon through the link, fall into a rabbit hole, and buy anything else after that first click through I get a piece of those sales as well. So if they are sent to my landing page by one of the authors, then click through the B2R link, and then click through to Amazon, then buy a blender while on Amazon, I get paid.