Fanfiction inherently limits the number of people who will ever look at it; an independent work stands on its own merits, but a fanfiction stands on both its own merits and the merits of the continuity to which it is attached. Write the best fanfic ever about Harry Potter, and most people still will never read it because your audience is restricted to Harry Potter fans who read fanfiction - a doubly restricted group.

While it is undeniable that it can act to promote your material, you are forever constrained in audience size by the above factors, as well as the composition of said audience by said people who consume fanfiction of fandom X.