From what I understand, channels can be used to extend Guix, but don't quite work like Flakes, more like the old Nix channels. The thing that makes Flakes special is that they allow you to treat Git repositories like software packages. No more need for a distribution to package the software, you can just drop a 'flake.nix' file into the Git repository and that makes it behave like a package that users can install and other Flakes can depend on. I am really fond of the ability to just run software straight from Git, e.g.:
nix run github:ggerganov/llama.cpp
Makes it really easy to switch versions, fork, patch or otherwise customize things.