What are the benefits of such a width range? Does it mean the user can adjust the thickness of the characters independently of font size, or something else?
That is already possible with just a weight axis. The benefit is a lot more flexibility in design and layout, plus just a preference. The Iosevka font also being discussed here today is on the narrow side, while Source Code Pro is slightly wider and Courier even more so. Inconsolata can cover all these and more.
