Technical considerations aside, I find rems have two practical benefits over pixels.
- The web is by and large a text medium. Relating sizing to text gives designers and developers a more direct connection to the relative scale of elements on a page (e.g, 1rem is “text size”, 3rem is “pretty big text”, 35rem is “compact paragraph length”, etc.). When we’re mostly trying to size text and fit text in boxes, a text based unit conveys proportion without the need for mental conversion.
- Pixels encourage fiddling. Anyone that works with designers is familiar with the “try it one pixel bigger. Now smaller. Now bigger” conversation (I admit I was guilty of this in my past life as a designer). Pixel fiddling wastes time, with no significant benefit to the end user or business. Rems are chunky units — they naturally guide people away from minute adjustments. In companies I work with we usually say “let’s stick to 0.25 increments for sizing and spacing unless it’s a really special case”. The time and mental energy saved with this rule have been more than tangible, especially when paired with elimination of modular scale or sizing variables.