If you're working on a laptop, you spend the majority of your day hunched behind a tiny keyboard and trackpad.
Even if you're working on a desktop computer, chances are real that your screen can't be positioned high enough to keep you from bending your neck or looking down to look at it.
Ideally, you'd want to set your moniter at a height that allows you to look at it straight, withoud having to bow your head or neck to look at your screen in the preferred angle.
After sitting hunched behind my laptop for several years, I've improved my posture by adjusting the place my eyes are focussed on. I've adjusted the height of my monitor drastically. This way I altered my viewing angle to a height where I no longer slouched. Now I no longer pull up my shoulder for hours and my spine/back/shoulders no longer hate me!