This turned out to be a bit of a ramble, but I'm kinda rusty, so sorry about that.
Apply Sturgeon's Law. 90% of all things are generally eh. The works you like enough to find fanfiction for probably are past your personal 10% mark, now you have to find the 10% of fanfiction that jibes with you.
What does the 10% have to offer you? Well, some of them might just explore certain aspects and perspectives unseen in canon, or they can be utterly transformative, entirely upending the work. The best of them are a direct improvement in all aspects, from characterisation to plot and worldbuilding, while either keeping to the canonical themes or going their own direction entirely.
In the end, fanfiction is just work that builds off a pre-existing work, and guess what? Most works are fanfiction, one way or another. Everything mankind has is built from older stories. Myths, folks tales, religion, philosophy. Dante's Inferno is considered a great piece of literature and single-handedly established the foundations of what would become the modern Italian language. It could also be interpreted as at least a bit of a self-insert fantasy if you shift it 45 degrees to the left.
Thus, I find it kind of erroneous to ask if fanfiction is good or bad- it simply is, like all works of literature. I'm aware of the reputation modern fanfiction has, but again, when you filter through the 100% of all fanfiction from the 10% of work you like...a good deal of them just won't hit that mark.
Even today new authors begin their first fanfic, and over the past few years at least some of them have authored original works.
Personally, having a world to build off and expand makes for good practice in worldbuilding, since as you already have a baseline, you are essentially forced to ask about deeper implications, as well as filling in the gaps of the world while trying to make it mesh with what is already there. And believe me, most original works simply don't reach that far in terms of worldbuilding as well. Not everyone can be Tolkien and set up every corner of the world, nor are they necessarily relevant in the main story even if the author did think that far, so there is a lot of room for new stuff. The world is your oyster.
Now, what I really want to know is what will the world have to offer when today's works become public domain? Now that'll be interesting times, though we won't be alive to see it.