Often obnoxious stuff like headers that follow you down the page are implemented in CSS. It also neatly deals with sites using gray on white text, absurdly narrow text columns, etc. It also neuters most cookie notices and similar BS that uses CSS to block visibility of the article until you remove it with ublock origin's element zapper. Instead of zapping things all the time on each new site I visit, I nuke it first and ask questions later.

I think it is also used for tracking people. https://github.com/Bogdaan/spycss

Or maybe he just likes content without flashiness and annoying sidebars, etc

Incompetent web design. CSS is a powerful tool of destruction, by taking it away from webdevs the web becomes much less broken.