There are people out there, namely Matt vs. Japan and That Japanese Man Yuta says that textbooks like Genki and Minna no Nihongo are useless or teach incorrect Japanese and you should just immerse. However, this might actually become harmful to your journey of studying Japanese. Let me explain.
So why do people advocate just learning Kana and start immersing in Japanese media? The idea is that if you immerse in native Japanese media, you will eventually learn the language over time. Sure, some have success learning the language through immersion by looking up stuff and figuring out the grammar, vocabulary, and Kanji. However, it can get overwhelming for most people and suck the fun out of learning. Not only that, it takes a lot of time through immersion to get to a level you can understand the language. Yes, many people try to read or play their favorite game very early on their journey and get frustrated. Also, this approach takes a lot of time as well.
The problem is that adults cannot learn a language like a baby by just hearing it because when we enter adulthood, our minds work differently. This is why apps like Duolingo and Rosetta Stone are very ineffective, as it takes a similar approach to learn a language implicitly and does not explain grammar rules. Language is not math, and grammar rules can be applied in many ways and broken too.
However, we can speed up the process, and that is through textbooks like Genki, in which people who advocate immersion say they teach incorrect Japanese. That is not necessarily true. Sure, the Japanese used in textbooks may use grammar in a way that it’s not used in real life. It’s to illustrate the usage of how the grammar point is used. In a way, textbooks serve as the foundation of the house.
Once you understand them, you can pick up how the grammar points are used in native Japanese media very easily. Unlike picking up grammar points, vocabulary, and Kanji through immersion, the work is already done for you in textbooks. You just need to study them. In a way, textbooks are the turnkey solution, while learning through immersion is the custom solution.
Another bad point about learning just from immersion as you might pick up bad habits and start talking like an anime character. You might use language that might be considered rude as characters in fictional works like anime, manga, visual novels, and video games talk in a very exaggerated way. Once you learn these habits, they can become difficult to break. If you understand the basics from textbooks, you won’t end up talking like an anime character.
That said, this doesn’t mean immersion should be avoided at all costs. On the contrary, you absolutely need to do it to reinforce what you learn and use the language. However, you should build a solid foundation before immersing yourself in native media. That means finishing basic Japanese textbooks first.
However, finishing lower-intermediate textbooks like Quartet I and II or Tobira as well can tremendously help as it has most of the grammar used in Shonen, Shoujo, and some Seinen titles, namely the stuff from Manga Time Kirara and similar magazines. Immersion, while important, is not a silver bullet to mastering Japanese for most people. For most, having a solid foundation of grammar rules, vocabulary, and Kanji can go a long way to prevent frustration and burnout. Also, you should avoid resources or pay for courses that advocate only immersion. There is no silver bullet to learning a language and a short amount of time. It takes a lot of time and practice to master a language.
Remember the saying, put the horse in front of the carriage. The same goes for knowing basic Japanese grammar and knowing enough Kanji and vocabulary to make immersion effective.
This video pretty much sums up why learning Japanese by just only immersing in Japanese media, namely anime is a bad idea for most people: