Its similar to a band starting off with cover songs. It gives a focus for practicing skill while one is still developing their own originality. Plus it helps an artist gain exposure by using an established name.
Yes and no. Yes, simply because any artistic practice is going to help you improve. And no, simply because you have the power and the imagination to create your own art, so why bother doing what everyone else is doing?
As long as the artist doesn't claim the design and the originality for themselves, they can improve their artwork skills and meet other people that carry the same interests.
Fanart also gives an idea of complex character designs, notions, personality, etc, and evolves appreciation of other artists. In all my life, I have never done real "fanart", only sketches of pokemon. Studying the art of creating characters can be difficult for beginners, therefore those who prefer drawing characters they admire should draw fanart so they get the idea of how these things work, but NEVER claim it for themselves.