Have you tried to read content on a site with fancy scripted fonts that were hard to read?

Or a beautiful soft color palette with poor contrast?

Or what about text that was so small that you had to squint to read it, and even then it seemed like the words ran together. Are the letters spaced too close? What is going on here, this is so annoying. I'm getting a headache just from trying to read this line.

Using rem units for font size rather than px ensures that your content is able to adjust if the user has a different baseline font size selected

Using px (pixel) measurement size for your fonts means that your font size will not automatically adjust for folks with a different baseline font size.