I bring you an example. I design and develop 6 (2 rows, 3 columns) grid cards with a text of X pts which are supposed to stay, for the sake of aesthetics, together in the viewport. After bringing the sizes to code I notice that the bottom cards are being cut at the bottom by let’s say over 100 px. (Because I have chosen font sizes based on screen instead of viewport). Yes multiple chrome users have different sizes of the top bar, but the standard is common and known. If I designed since beginning with the standard viewport, I could guarantee that most people (Pareto’s Principle) could access to the design EXACTLY how I wanted to.

It is not a huge issue, but by just considering viewports (and learn to manage littler spaces), you can design easier products immediately. My background is being a coder, and receiving a design which is just an idea I have to transform into something correct (like changing font sizes avoiding overflow) could be frustrating especially when I have thousands of other things to do.