I can see this being useful for creating pleasing ratios between font sizes. Though I feel this may be less useful today than in the past. Headers, navs, and other elements often require responsive font sizes to adapt well to all screen sizes. You could still maintain a ratio if using a container query of the full document, but it's usually not a good idea to scale the body font more than a couple points up or down for readability.

I think this tool may be good for documents with careful typesetting and layout, but maybe not modern, interactive, responsive pages.