I'm a bit confused with the Heavily Obscured condition. It basically says if you're heavily obscured, you are essentially blind. So why do you have to make a Dex check to become hidden if your opponent is blind? I can see it if you're making a lot of noise, but if you're hiding at the end of the hall and not doing anything, would you still need a Dex check?
Because the DEX (Stealth) check is really checking to see not how "unseen" you are... but how quiet you are, how much you are masking any possible scent you have, and how much you aren't affecting the environment.
The rules are set up that you have to
start by being unseen-- either behind a solid object that entirely blocks line of sight, or in an area so dense with something (foliage, fog, darkness) that you essentially
are behind a "solid" object due to it being Heavily Obscured terrain. You start here at the barest minimum. If you don't have this, then you can't even bother trying to make the check, regardless of any other factors.
Once you've established that you've reached the requirements for not being seen clearly-- IE behind Heavily Obscured terrain or out of line of sight-- the check is then to see how much of the other masking agents you put into place so that nobody knows you are there. The higher the check, the quieter you, the less movement you are making that causes dust to swirl, or branches to sway, or tapestries to flutter, or shadows to dance on the walls, or scents to waft, or air currents to breeze. All the various things that someone could notice and give a hint that someone is around. The lower the check, the worse you did to mask those things and which an alert person might notice.
It's very counter-intuitive to a certain extent, because everyone I think goes into it believing that "hiding" is all about not being seen, and thus if someone can't be seen then they're hidden. But the game takes it on faith that
of course someone trying to be stealthy is trying not to be seen. That's a given and assumed that anyone good at stealth is going to be good at not being seen. But that it's all the
other stuff that we usually don't think about (sounds, smells, reflections, shadows, air movements etc.) that are what the perceptive person will pick up on to realize someone is around. And so that's what the DEX (Stealth) check is determining a value for.