As Janet said, BookScan provides accurate sales information. But because access to it is limited, bestseller lists are what provide publicity. Readers have no way to hear about what's hot on BookScan, but they can check the lists as they're published.

Actually, this whole bestseller thing is odd to me, the way the report you get on the Monday TV news about what movie grossed highest over the weekend is odd. As a reader/viewer, I'm much more interested in a review that would tell me what the thing's like, to help me decide whether I want to read/see it, than I am to know which book or movie the most people chose this week.

As a writer, of course, I have a crasser interest...