And to add, it also is a HUGE help to authors. There is a huge problem in the book industry with"winner takes all" within Amazon and other big book websites. It means we as readers are getting less and less exposed to books that we would like but fail to get noticed. The more recommendations and distribution of unique recommendations the better.
I think most people who recommend a book do it to help the world and spread knowledge and would be glad to have their recommendation reshared.