1. cross-platform.
2. allows users access to the latest version of the software immediately.
3. allows installation of multiple versions. API/ABI stability is great but perfect backward compatibility is never possible. E.g. some apps actually rely on bugs in the platform and if you fix them they break; yeah yeah I know the software must be fixed but there may be a ton of reasons why that doesn't happen or at least not happen immediately and there's a clear need among users to be able to install multiple versions of a platform/library.
RPM/DEB theoretically support (2), but in practice distros never do that. Third party packagers that immediately provide the latest versions are most exception to the rule because they need to replicate their effort multiple times over multiple platforms thanks to the lack of support for (1). RPM/DEB don't even try to support (3).