The main benefits of using royalty-free music in a project are cost and convenience. For a flat fee, the music can be used over and over by the purchaser. The purchaser does not have the right to sell the song to others however.
Royalty-free music is used in projects like feature films, advertising and educational videos. The use of non-royalty-free music for such projects requires licensing and complicated payment structure--and there are serious legal implications for copyright infringement.
Royalty-free music is often considered generic. Popular hit songs, or any song recorded by an artist for musical distribution, always require paid royalties. As a result, although there are huge stock libraries, royalty-free music can be limiting.
Royalty-free music is not actually free; there is a flat cost paid. The "free" applies only to royalties. There are, however, some distributors of royalty-free music that offer free songs. Always make sure that any stock music offered outright for free is legal to use.
Royalty-free music is not the same as music in the public domain. Public domain means that no one can claim ownership over it; it can be used without permission or payment. Royalty-free music does have copyright holders--they just sell the use of it.