I think that writing fan fiction can help a person to get published, by providing a practice area. One way to improve as a writer -- perhaps the best way -- is simply to write, write, and write some more. And getting good, critical feedback is an important part of the process. 'Real' authors, or aspiring ones, pay good money to have their work edited (beta read) or attend writers' workshops or take creative writing courses. Fan fic writers can get that for free, and learn how to improve their writing.

And, I think that one reason why former fan fic writers take their fan fiction off the internet when/before they get published is so that no one can officially, legally point a finger at them and say that their published book uses characters that were created by someone else (i.e. that they have just taken one of their fan fictions and changed the names).