"Pixel perfect" is about attention to detail and consistency. Margins, padding, or the combination of these inside other containers will stick out when they're not consistent.

Here's an example that I personally encountered: what if you have a <h1>Text</h1> and it has a certain left margin. Then another heading except it has a nested button component (which internally comes with some padding). Then the "Text" in both aren't aligned from section to section and it is jarring.