What's the problem with writing something obvious? Honest question. I don't see why that should matter. (I mean, sure, you don't want to write 1000 words about how you should blink your eyes so they don't dry out.) Try to think of one person you know that wouldn't already know what you're writing. Even if it's obvious, it's not literally something everyone in the world already knows or shares your perspective on.
> What's the problem with writing something obvious?
You're not contributing something useful if you're just pointing out the obvious. Yeah, you can write the ten millionth article on how to change the default slogan in WordPress, but unless that's the maximum extent of your knowledge, you're wasting your potential and everybody else's time. That's how I feel about it at least, and why I don't write stuff that's too obvious or has already been widely covered. In that case, I might give a small overview and some tips to newcomers and link to others that I think did a good job.
There isn't a lot of a difference to me: the obvious has been stated by nearly everyone. You're right that there will always be some that can benefit from what one might consider obvious, but the amount of people depends on the level of what you're writing about. There are people that don't know how to change the default slogan on a WP site after all.
But don't get me wrong, I'm not trying to say that nobody should write about that, I'm offering my perspective on why I probably wouldn't.
