We obviously put margins on stuff, but never at the body element for many reasons. Padding would be better for example. The margin only makes sense if you don’t apply any further styling.
if you have h1 with top margin 10px, and body margin 8px, it "adds" up to 10px, not 18px.
Once you write article-based sites, without a random navbar that people don't want, then it can become quite nice