The use case is quite simply for re-trying a build after a partial (possibly transient) failure, where multiple artifacts are "uploaded", not just NuGet artifacts.
The point is you still cannot (and must not) overwrite any NuGet artifacts with a given version, but you can skip them, which makes the build much more flexible.
dotnet nuget push added the --skip-duplicate option:
When pushing multiple packages to an HTTP(S) server, treats any 409 Conflict response as a warning so that the push can continue. Available since .NET Core 3.1 SDK.
This works as expected with nuget.org, where trying to push a duplicate package version returns a 409 Conflict and is skipped without breaking the build.
This does not work with ProGet, probably because this option is fairly new?