This could be helpful for development / building releases where you no longer have to maintain multiple repos for stable/testing and so on, having apt point to a known stable snapshot
I doubt that. I doubt that Debian is going to abolish sid; testing repositories.
At some point, a developers-only or testers-only package needs to be uploaded. If there’s only 1 repository, where do I upload it?
Once users run apt-get update and apt-get full-upgrade, they would get the experimental package.
During builds, --snapshot can be useful. But users won’t be using --snapshot. That’s why repositories for testing cannot be abolished.
This is an interface to https://snapshot.debian.org, which might be handy (and I guess which was the reason for --snapshot to be invented) for reproducible builds.
Whonix once upon a time was built using “frozen sources” as it was called from snapshot.debian.org.
Advantage is better built stability. A stable git tag cannot break (unless snapshot.debian.org issues) because some dependency changed in Debian stable (which is fortunately rare).
Also helps with reproducible builds or at least verifiable builds.
Disadvantage is that before making a new point release, the snapshot URL needs to be updated, which is development overhead.
Another disadvantage is that later custom user made rebuilds are still using outdated packages. So before there was an option --current-sources versus --frozen-sources.