What are the best client-side JavaScript application frameworks?

Angular uses the existing HTML structure and builds on top of it instead of requiring you to learn a new templating language. Because it's just vanilla HTML, it is more familiar, and easier for beginners to learn. Directives let Angular know which HTML elements are under its control, and how to use them.

Being directly on the HTML it's more transparent what's going on, and it's possible to get a good idea of what the page is doing just by looking at the template.

Also, it makes embedding possible, as you could have an angular app within an existing site so you don't have to rewrite everything at once.