Is marketing must-have skill for developers?

Yeah, definitely. My biggest mental hurdle with marketing is that it doesn't FEEL like productive work. With code, there's something to show for it right away. With marketing, it feels so abstract.

At one of my least-favorite jobs over the years, the highlight was a crash course in the management process they were planning to implement across the company. The big take-away (for me, at least) was that everybody's job is sales. Specifically, any work that doesn't help an actual sales representative get money out of more customers is work that the company would rather not pay for.

Once you start working from that perspective, marketing becomes critical, because you can't make a product more valuable without knowing whose valuation matters. Even with routine tasks like refactoring, it's worth asking if the new flexibility will really enable anything that prospective customers will care about, or if the work is just because it seems like the next thing to do.

Design is trickier (as you probably already know), because there are elements that are qualitative enough that they should probably just be enforced by development environments. Then, there's a handful of guidelines that let someone fake their way through a project when a designer is unavailable. And then there are two bottomless pits of "everything is subjective" and "this could always be better" that (in my experience) definitely require professionals. But putting together something that looks decent should be a priority for all developers, I think, and a willing developer can get pretty far before needing to hunt down a professional who understands the project.

Yeah for sure. It's very common advice to developers that they need to focus on marketing first.

I've made this mistake so many times already. This time I've been focusing on marketing first instead of the product.

There's definitely a balance though.