It is a mark of professionalism, at least for the shared paradigm of the journal or book you review for. A reviewer must navigate the distribution of current opinions that surrounds his paradigm. Sometimes the editor may place constraints on the merits of an article assigned to a reviewer, besides imposing a desk rejection. Why must this be so? One reason is that there is usually a journal for every paradigm. It is not unknown that many scientific articles were rejected by major journals, and that later got acceptance in a more general-interest or field journals. Two outstanding examples are Robert Lucas Jr article on Rational Expectation, and George Ackerlof article on hidden information.
You learn how to evaluate other people's work better
You can add it to your CV
You can gain favour with the journal editors, which might help you at some point
You become more recognised in your academic community
You get a good idea of what quality of article is required for publication in that journal (e.g. high ranking journals with many submissions might require minor corrections from all reviewers)