As I understand it, the point of these attestations is that you can see what goes into a build on GitHub - if you look at the recorded commit on the recorded repo, you can be confident that the packages are made from that (unless your threat model is GitHub itself doing a supply chain attack). And the flip side of that is that if attestations become the norm, it's harder to slip malicious code into a package without it being noticed.

That's not everything, but it is a pretty big step. I don't love the way it reinforces dependence on a few big platforms, but I also don't have a great alternative to suggest.