Not a front end engineer but I imagine this boilerplate allows the JavaScript display engine of choice to be loaded and then rendered into that DIV rather than having any content on the page itself.

It's because "modern" web developers are not writing web pages in standard html, css or js. Instead, they use javascript to render the entire thing inside a root element.

This is now "standard" but breaks any browser that doesn't (or can't) support javascript. It's also a nightmare for SEO, accessibility and many other things (like your memory, cpu and battery usage).

But hey, it's "modern"!