If you believed them then you'd think every book published is, like, really amazing. From The Guardian:
There's a lot of received wisdom in the publishing world – for instance, if you write non-fiction, your book needs a subtitle. Never mind that fiction doesn't require that extra bit of explication (Crime & Punishment: Murder and Redemption in the Empire of the Tsars anyone?) if you write non-fiction you simply must spell out what you're up to for prospective readers! This may be a wise policy or it may be nonsense, nobody knows.
Then there are blurbs, the more of which you can plaster on your paperback the better. Do these blurbs – many of which could be transferred from book to book without great difficulty – actually sway readers? Usually these are from newspaper reviews reduced by your sales people to a string of superlatives here, a comparison to somebody more famous than you are there. If the blurb comes from a review by a famous person, then they may just run with the name of the celebrity alone ("The Da Vinci Code is f*cking awesome!" – Salman Rushdie).