I understand how em and rem work. I’ve just never had an instance where in terms of design I found changing a base font size was adequate for a site-wide font size change. There would always be spots that the font size had to be tweaked. I prefer to use pixels and media queries to ensure the site delivers the exact UX I want and the screen size in question. I think ems and rems are holdovers from the days when browsers couldn’t scale the website.