that would only be a side effect of its main purpose

In addition to being an unambiguous semantic marker in the HTML, the download attribute can serve as a simple and elegant styling hook. CSS attribute selectors—code that lets us create styling based on the qualities that help describe HTML elements—allow us to target any link that is a download and style it without having to attach a special class:

This is not an MDC Web component. It is an addition that SMUI provides.

Bonus