It's just how straightforward it is. With GitHub's fork-then-PR approach I would have to clone, fork, add a remote to my local fork, push to said remote, and open the PR.

With agit flow I just have to clone the repository I want to contribute to, make my changes, and push (to a special ref, but still just push to the target repo).

I have been making some small contributions to Guix when they were still using email for patches, and that (i.e. send patches directly to upstream) already felt more natural than what GitHub propagates. And agit feels like the git-native interpretation of this email workflow.