It's not about finding applications, it's more about the ability to have multiple versions of the same package installed and used concurrently. Best you can have with FHS is something like /usr/bin/python3.8 vs /usr/bin/python3.9, and this requires package maintainers to manually support this. Most packages don't do this, so if you happen to need - just a random example - an ancient Firefox version installed separately, "traditional" solutions like rpm or dpkg won't really help you.

Plus, a different organization of the filesystem, where everything that belongs to a package is stored under the same base path, rather than being spread across the whole FS. I don't really remember GoboLinux ideas, but IIRC they had something about leveraging use of the filesystem as a part of package manager.