As a purpose-built tool, it improves on the experience of having multiple windows open. While the headline feature is keeping them all in sync, for my particular app, that's not as big a deal (though it's pretty cool that it will even do that for stuff like tweaking CSS styles in the inspector).
I've been playing around with it for a few minutes and I think what I'm really appreciating is that it's filled with dev tools of all kinds and it's really optimized for working on web sites rather than browsing. It can automatically open panes based on CSS breakpoints, and it has presets for many devices. Some of it is things I had in Chrome, but better, like rulers and guides and grids. Even the way screenshots are implemented shows they put thought into saving time and hassle over a thing you could of course do before with a few more steps. And it's not all related to layout: it shows meta tags and icons and previews for social media sharing.
Anyway, it's pretty cool IMO and I'll probably end up buying it if I keep working on web apps. The only downside I've noticed is that it feels a little sluggish, even when not heavily loaded down (e.g. just 3 panes). I'm using this monster M2 Ultra Mac Studio so it's a bit unusual for a browser to lag.