"Blurbs" do sometimes have an effect. Soman Chainani (author of School of Good and Evil and the other books in that series) has written that his first book nearly tanked. He'd been trying to get a blurb from a well-known author and failing at it. (And yes, that does seem to be something the publisher should be doing, not the author.) Bookstores weren't ordering in the numbers they should have been. But at the last minute, Chainani got a blurb from RL Stine. Immediately, pre-sales picked up, the book did reasonably well, and Chainani got a Netflix movie deal.
I don't know how typical that is, but apparently, some of the bookstore buyers are very influenced by the presence (or absence) of big-name blurbs.