Fan fiction is how I got into writing. I was writing enough wholly original characters that things had taken a huge turn away from the source material. It was different enough that it was obvious -- I was using a previously constructed universe as a crutch. So I tossed the crutch.
I also decided I wanted to be paid for my efforts. I don't make much from writing at all. But at least I can. I own the rights!
Fan fiction is a great entry into wholly original writing. But I would say, it's harder for me to world build because I have much more experience in writing dialogue and character creation and building.
And the following are fan fiction, after a fashion: Romeo & Juliet (from Tristan and Isolde), West Side Story (from Romeo & Juliet), Clueless (from Emma), The Inferno (from the bible), 10 Things I Hate About You (from The Taming of the Shrew), Cinderfella (from the folklore tale Cinderella), The Lion King (from Hamlet), etc. etc. etc.