Do book reviews matter?

This is an important question for all authors (for readers, see below). Another one is: Who writes reviews? The two questions are linked, of course, and both need to be answered.

I’ve always offered a free ebook in exchange for an honest review (print books are costly, especially for outside the US), and I thank all those good people who respond to that offer, whether directly or via NetGalley. But do those reviewers get more out of it than I do? That’s not a question reflecting a selfish quid-p...