REMs are useful almost anywhere size is explicitly set.
With rem, all font sizes are relative to the root element (aka, the
html tag). The reason for this is to make it easier to scale up or down for devices. You could technically change the html tag to a smaller or larger size to scale all font sizes equally – which is a super nice feature.… [T]he main thing to take-away is everything is dynamic and
relative to the root HTML tag.For example, change the html css font-size to a different number …
and watch how the entire grid adjusts and scales.
Source: Scotch.io