This makes it easier to use components in designs for different screen sizes. And then you don’t have to calculate every time what 40% or 60% of the width is.
This also makes it easier to design scalable applications that grow with the width of the screen.
I have this text block component with a few nested auto layout containers, one of which is the body below the heading with padding on either side. Yes I have it flexible with “Fill container” and would much rather set a percentage width on that body, like anyone would in real life.