Reviews help sell books--it's that simple.  Good reviews do a better job than bad reviews, but even many bad reviews have redemptive assertions that may benefit the book in surprising ways.
Reviews of your book normally come in two forms, pre-publication and post-publication.  Pre-publication reviews are something your publisher works hard at obtaining.  There are several reviewing organizations that have a stake in pre-publication reviewing, especially where library sales are concerned.  These would include, primarily,
Publishers' Weekly, Booklist, Kirkus Reviews, and Library Journal.  Libraries and bookstores and television shows, for example, have to do a good guessing job on what they feel people will want to both read and buy.  A good review by one or more of these reviewing journals is therefore highly desirable--though there isn't much you can do to help this process along, especially as a new writer.
Publishers' Weekly identifies those books that are most marketable on a national or mass-market scale, attempting with generally reliable success to predict what's going to be most popular.  It also gives you all the publisher's publication specs, such as how many copies, publicity budget, promotional tours by the author, and so on.  There are also magazine articles about the publishing industry that are of some general interest.
Booklist is aimed more at librarians, and tries to identify and review new titles in specific subject areas.  Reviewers for this journal are subject specialists.  The idea here is not necessarily to identify what will be financially successful, but rather what are necessary or exciting new additions to particular fields.  Booklist also creates "Best of" lists, such as best reference books, best cookbooks, best science books, and so on.  It also reviews foreign language books, the only major journal that does so on a consistent basis.
Kirkus Reviews is similar to Booklist, but its reviews tend to be more critical and often take a more academic approach.  The tastes here are more scholarly, even when addressing otherwise popular titles.
Library Journal is, again, aimed at librarians, and is the least critical of the major journals.  Reviewers here are not necessarily reviewing materials that fall within their area of expertise, but rather are working librarians who contribute reviews to the journal.  The reviews in general are more positive, as these reviewers are often generalists.

Post-publications reviews, however,
are something you can help with, at least a little.  These are the kinds of reviews that are more immediate--The New York Times Book Review, for example--but which also include those that take some time, even years, to catch up with your book.  This would include all the literary journals that carry reviews.  Getting the word out to these places is something you can help with.  Mention your book, for example, every time you submit something; put it in all biographical material; think about letting those journals where you have found some repeated success know that the book is out.  Your publisher will help you do these things, but make sure you are part of the process of helping yourself when it's appropriate.