Since I’ve easily spent more of my time as a writer writing fanfic than original work, I’m not gonna shit on it and call it cringe.. bc the real cringe is adults bullying teenage girls for having interests. That being said, there is room for nuance and criticism around how fanfic and fandom spaces affect authors and the literary scene. These critiques are angled at other hybrid authors of both types who may be thinking of doing more original work.

Good Things About Fanfic

• Easy for beginner writers: the characters, settings, and backing plot are all ready to go when you first sit down. Obviously, you already care a lot about them and it’s quite easy to build off of this. If you haven’t already come up with a premise for your fic, common ones abound: high school and college AUs, coffee shop AUs, etc.
• Easy to publish: Not as in book publishing of course, though that happens, but as in posting web fiction. There are several sites that get massive traffic and host fics for even very obscure fandoms. There is no gatekeeping to what quality is needed to publish and it is easily to release to a wide audience. Trope searching also lets people know exactly what type of fic they’re getting into.
• Easy to find community: it’s really easy to get interaction with other fans who get invested in your fic.
• It’s a valid form of art and way of enjoying something. (You might think that's dumb to point out, but as a teenager I really needed to hear this)

Bad Things About Fanfic

• Repetitive ideas: the soulmate/hanahaki/high school AUS can only go in so many directions, guys.
• There’s a lot of really fucking creepy erotica written about real ass human beings.
• It’s harder to get engagement as an original author. Yeah, there are spaces focused on original works, but if you’ve spent ANY time on a creative side of the internet, you’ll notice that fanworks dominate them. If you post as a hybrid creator you get noticeably more engagement on fanwork than original work, simply because more people are looking for it.
• Trope searching has negative effects on browsing original literature. By trope searching, I’m referring to the website design feature fanfic websites have where you tag the main tropes, eg. Enemies to Lovers, BAMF!Character, Character Needs a Hug, etc. This doesn’t translate to an actual narrative. Summaries for original work are its themes, its premise, not just word salad. If you’re going to write original work, for the love of fuck don’t try doing this stuff or I’ll try jamming your hand in a door.

If you mostly read/write fanfic, why branch out?

Of course it’s fine if you want to only read and write fanfic, I can’t stop you lol. But if you have any desire to create original work, then reading and writing original fiction is an important step to creating strong original work. Fanfiction is ultimately, a genre, and to be a strong author in any genre, you have to read it. Ideally, you can draw from a wide net of sources, including art that doesn’t neatly slot into any one genre, but you should at least read in your dominant genre. For example, if you want to be a horror writer, then some lateral research can help you figure out what tropes are most dominant, what techniques other horror authors use that interest you, and room for ideas that haven’t been very explored.
It's still ok to have a narrow scope in taste and create for that narrow scope. A lot of people really thrive off of that. it’s also okay to not have one standout favorite genre! Genres aren’t blood types or Harry Potter houses, you’re not bound to one and one alone!
There are a variety of goals in reading literature. The point of fanfiction is, to put it bluntly, self-gratification. Whether or not that’s as obvious as a self-insert erotica, that’s what it is. But the wider form of literature has greater points in reading a book: to make you uncomfortable, to shock you, to give you a new perspective, to empathize with someone you thought you couldn’t, etc etc. Fanfiction doesn’t give you any of these, and can’t give you the full range of experiences in reading.
Also, exposing yourself to new forms of art will probably widen your tastes, especially if you’re not into it exclusively to be in a fandom. Plenty of great books just don’t have fandoms, either because they’re niche or just not very fandom-able (Kafka, for example). If you participate a lot in fandoms, chances are you only consume fandom content and commute from one to another via that space. If that’s your only way of finding new things to read, I guarantee you’re missing out on pieces of art you would enjoy.