I think every developer needs a little knowledge about marketing🤔. Do you agree with me or not?

I think every person can benefit from learning a bit about what makes other functions in a business successful, even if there are no immediate plans to use that skill. Building relationships is a good idea, and you never know where learning new things will take you. To answer your question, most definitely and I'd add sales to it as well. If you ever want to build a product that is used, these 2 skills are highly useful.

If you're a developer and launching your own product, yes. Most developers aren't founders though and developers at large companies don't need to know anything about marketing in my opinion.

In my opinion that depends a bit if you're a generalist or specialist (and team size). If you're a developer at a large firm working a specialist role I don't think it's of that much value to learn a lot about marketing (at least for your role in particular). If you're in a generalist role in a startup, then definitely. But for startups that goes both ways (marketing, sales, CS, etc should learn enough development to be able to manage user expectations and plausibility of feature requests and improvements).

Yes if you bring something new online, you really have to learn SEO, people who are Full Stack Developers do not mention the SEO factor, but SEO is your content-image tuner to Google 🧐 Every page before you start is in default-mode, the tuner tunes it in so Google understands straightaway. Optimise the page incorrectly at the start of before entering and the page will fail badly.

Perfectly said! I would consider myself a "specialist" and to me to learn some marketing at a beginner level was, and still is a nightmare. I think the ideal scenario would be a specialist in product making, a generalist, and a specialist in marketing! The generalist would be the between these two sides of the spectrum to communicate what's supposed to be done. But for a solopreneurs, that's rough and it's nearly impossible to find anyone willing to join a startup these days. A little bit of knowledge about marketing won't take me far, the competition is full of extremely aggressive and talented marketers, not sure it may do anything. But I guess making a great product may compensate for my lack of marketing skills.

Depends. If you're making something and plan to publish it, I think yes, you need to learn/know.