YMMV. I think there are benefits in uses such as video and animation, having letters you can easily morph is good there. I think a lot of the benefits in web design will be on crazy avant-garde stuff like some of the things Nick Sherman's done for Typographics, websites where it feels like the type is bubbling and twisting in front of your eyes, maybe uses that don't even exist yet.
If it's more conventional web, document and poster design, the particular benefit for my view is optical size options, you can have one file for all sizes. Google Fonts' article introducing Roboto Flex shows what you can do particularly well. They use a standard setting of Roboto for the body text and different variable font setting for the heading: by pushing up the x-height, stretching out the width and slimming down the weight it morphs into feeling sixties sci-fi, like the end titles of The Incredibles. It's really quite striking, it feels both distinctively different to the text and smoothly compatible with it.
For posters, I guess the benefit is being able to jam a big bold headline into exactly the right space, and maybe to crunch the width a bit if something changed at the last minute and you needed to scrunch in a bit of extra text. Wood type was sold in a huge range of widths for exactly that reason.
Often though designers, even ones who care about niceties like optical sizes, want a hierarchy with a separate typeface for each use: a display type for headings, a serif or neogrot for body text, a separate agate/caption font for small text, etc. These could all be catered for by one VF, but often you'd rather have different typefaces for them. We're not in the sixteenth century where the expectation is a Garamond-style serif at every single size and for every single use, although if you like that classical style you could do it with VF and make it look very very good.
For websites, compared to tracking garbage and images, the font size is a very small part of a website's payload, not very important.
The really irritating thing about some of the more complex variable fonts, at least ones I've seen, is that they haven't been marketed with a quick set of recommended styles that look good. That was my gripe with the big Google Fonts demos, Amstelvar and Roboto Flex: sure, everything is parametric, you can pull the ascenders longer or the descenders longer, but which combinations of settings look nice for what uses? It would be nice to see a quick highlights reel of nice combinations of settings.