Basically, It can be used to dermine your display resolution.

It’s worth noting this permission can be used for legit reasons sometimes, like for rendering some graphical elements. But most of the time it’s used for fingerprinting your browser. And it’s so widespread, you can see it everywhere from government websites to Linux distro websites. Crazy stuff.

I would direct you towards the same question on Tor StackExchange. I think the Tor Project documentation explains the problems with canvas fingerprinting quite nicely. Hope it helps understand the risks and the purpose of this permission.