[WARNING: ESSAY ALERT
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Hey there! Traditionally published author and illustrator here
I can't speak specifically for comics, as I haven't had the chance to publish one yet, but perhaps I can offer some insights as far as traditional publishing goes, too.
I already spoke about making a living with traditional publishing in a previous post: the tl;dr version would be "nope, being traditionally published doesn't automatically mean that you're going to make a living out of it".
Again, I'm saying this as someone who lives in a country where royalties on books are ridiculously low (6% of the cover price) and where an increasing number of small/medium small publishers won't even give you an advance payment on your contract.
Even with "big" publishers, the advance payment isn't that high at all and will barely cover one month worth of salary... and since you can't publish more than one book a year with the same publisher, you can easily imagine that the money you're going to make out of it won't be much XD now, of course this is going to be different for comic artists with ongoing monthly/weekly series, buuuut... judging from the sheer number of comic artists who also do freelance work as illustrators to support themselves, I'd say that it's pretty clear that comics alone aren't enough to make a living here. Even the "big names" of graphic novels here usually rely on stuff like TV shows, animation, advertising, illustration, workshops/private lessons or even Patreon to make a living. Authors who can truly make a living solely out of traditional publishing are very few (and usually skew older: most artists I'm aware of who are strictly comic artists are people who've been in the industry since the 70s-80s).
That said, I also agree with what @Caro said about the fact that you don't necessarily have to make middle grade work to gain money. Many of our young comic artists who recently gained popularity actually make YA or even downright adult rated stuff. See Zerocalcare, Mirka Andolfo or Loputyn.
So, if it's not the money, what does traditional publishing have that webcomics don't?
I think both Caro and @Kaydreamer made some really good points.
Legitimacy definitely does play a big role: many traditional publishers still side-eye self publishing and webcomics as something only "amateurs" do. Just recently I stumbled upon a competition for "young comic artists aged 18-35"... and they STILL demanded that you had traditionally published work in order to participate. Cons and events won't really give any space to webcomic artists or platforms, either. We had a grand total of one (1) traditional publisher creating a platform that was similar to Tapas/Webtoon in order to find more works to publish... but even then, as soon as they settled on a bunch of names to publish, they stopped caring (platform is still there, but kind of a wasteland nowadays). So yep, there's definitely this (rather backwards, if you ask me) concept according to which "you're not a real author until a publisher says you are".
As for things that traditional publishing does that webcomics can't (yet), however, I'd definitely want to add something about:
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More opportunities: traditional publishing -when done well and with publishers that actually do their job- also gives you an easier access into events such as presentations, book signings and cons, thus allowing you to meet more people and get involved in more projects. My co-author took part to many events and got to meet movie directors, journalists, editors and more. Heck, one of these connections got us the opportunity to talk about our book on TV once
granted, it's not all sunshine and rainbows: many of these events will be unpaid and you WILL meet plenty of buttholes, but it's definitely easier to get into these things when you have a publisher behind your back. Some publishers will also send your published book to bloggers and book reviewers, who'll then write articles about it, thus contributing to spread the word about it. In short: no more need to self-promote.
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Printing and distribution: printing a book/a comic can cost quite a lot, especially if you're working in full color, have a long form comic and/or want to have a fancy cover. And once you're done with the printing, you have to physically send it to your buyers. Traditional publishing takes care of all that for you, so you just have to sit there and enjoy the ride.
As far as having an agent goes... again, I'd say it wildly depends on the agent and the country. Agents where I live do... basically nothing, tbh. And no, you definitely don't need one to publish, although if you're just starting out you may want someone with a certain degree of experience who can help you with publishing contracts to avoid scams (doesn't have to be necessarily a literary agent, though, sometimes even a fellow author can help with that).
Also, re: editing... it really depends on the publisher. Some publishers will do little to no editing at all. Others will try to make you rewrite the whole thing from scratch (sometimes with hilariously bad results, such as suggesting entirely wrong verbs, or making you change the food your character is eating in a minor scene because... the editor doesn't want your character to eat pizza, apparently?
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Long story short: it really depends on the publisher, the kind of content you make and your personal preference. Neither option will instantly allow you to make a living out of it and making it big is incredibly rare in both situations. Self-publishing definitely gives you more control over the stuff you do, but it's also true that you're on your own unless one of the bigger platforms decides to feature your content (in this sense, platforms aren't that different from a traditional publisher, tbh). Traditional publishing may not offer as much freedom, but it definitely gets rid of a lot of the issues that come with self-promoting and self-distributing. Both allow you to meet your audience, however it's definitely true that webcomics allow for more frequent interaction (which can be good or bad depending on the feedback/community surrounding you), while with traditional printing you only get to see your audience at meetings, which usually happen far less often.
Most importantly, though, neither is perfect and both come with challenges and frustrations of their own.