I think fanfiction gives life back to characters when the series is dead. I mean sometimes you just look at a movie and think I wonder what was going on his head during this part and you find a amazing fanfiction with a author's opinion on his emotions and it can just complete it.
I do not think it is lazy, I think there comes a point where authors feel trapped because they made a awesome story that could have possibly be published but can't because it is a fanfiction although 50 Shades of Grey started as a fanfiction...
I think it can gain confidence in writers and it can get them to learn how to develop characters and get the foundation of writing and finding what people are interested in and not.
I don't think there is any right way to become a professional author. Some dive right into original fiction and some start with fanfiction, and some authors do both. It is just another pathway to take and see if you are even interested in writing. Some have written fanfiction and realized they love writing and some started and decided they hated it.
For me, I started with fanfiction and I still write fanfiction even through I am also writing my first novel and original short stories. I find that sometimes it is refreshing to work on a fanfiction instead of your original work. It gives you a break from it and gives you something else to focus on. I read a lot of fanfiction, mostly to find the diamonds in the rough and I don't think it is wasting my time or the writer's time but that is just me.