There are a few fleshed-out articles about why utility-first CSS is a good thing. However, if you're just a little curious and want a brief introduction, Chris Ferdinandi has your back.
Utility classes help keep CSS DRY.
The stylesheet for my website is so small that I can inline all of my CSS still send most of the page in a single HTTP request.
Read the rest of Chris' article on gomakethings.com.