> On one hand, people say that you need to build to solve a problem. You find a problem in a niche, say accounting for plumbers, and build for that, then you just go and market to these people. On the other hand, I see people who advocate for "I just built something that I needed, and it got traction". There are multiple examples for both camps, but the problem is that they contradict each other.

Where is the contradiction? You do need to solve a real problem, and it's very convenient if that problem is one you yourself have.