1em is your base text size (what your content should use), >1, <1 when you want larger or smaller text, point being you don't dynamically adjust and break the users font size configuration.
Except that's exactly what you want e.g. when you want something to stand out relative to its context. For instance, you want your headings to be 150% the size of surrounding text, so when you have an info box with smaller text and a heading, that heading should not end up as tall as your main text headings.
Of course you could also have distinct heading classes or equivalent selectors for different contexts, and specify every case in rem, if that's how you like your stylesheets.
For example comparison with the OP's site:
- dynamic on : https://files.catbox.moe/qwa2az.png
- dynamic off: https://files.catbox.moe/glu3ud.png