"Loved it!" - Adolf Hitler
As such words are wont to do, "blurb" has evolved into a verb, meaning "to produce a short statement of praise for use in a publicity instrument".
Everyone knows that only egghead bookworms actually read all the way through the book reviews in the paper. Anyone who knows anything about the media knows that the people want sound bites. Of course, only a small percentage of the people who pick your book off the shelf (or search for it at Amazon.com) will bother to turn the book over and look at the back cover; fewer still read the blurbs. But cover blurbs are a form of advertising that publishers find irresistible. There's no way to prove that a cover blurb never influenced a potential bookbuyer's decision.
It's not enough to produce a sound bite of praise about your favorite book. It has to be associated with the right name. If a blurb is attributed to Joe Shlobotnik you've just wasted valuable cover space. And if it's attributed to Osama Bin Laden you're likely to have unpleasant consequences. Well, unforseen at any rate, and you know how publishers hate uncertainty.
So naturally, book agents and publicity flacks will send advance copies of a new property around to book reviewers and well-known authors in order to shop around for the perfect snappy blurb. But the same rainmakers have become careful about allowing their names to be used in publicity. These names are valuable, and so using one isn't free. Fortunately, that's what agents are for.