In order to make sure you actually have the freedom to modify the software, you want packagers to have the ability to rebuild the package from source - and preferably using just their OS as a build environment, not a black-box build environment from upstream like a Docker configured just so and make sure they have an equivalent binary. Even if there's nothing to patch today, there might be something to patch tomorrow.
I have seen many times Debian packagers try to build something from upstream and find that it just does not build anywhere other than the maintainer's computer. The fact that Debian requires that every package it ships can be built buy anyone in a generic environment is immensely valuable to free software, even if nobody used the binaries that Debian built. (And to be clear, other distros do the same, I'm just most familiar with Debian.)
I'd agree that in an ideal world, all the patches would be upstream and the binary would not just be equivalent but bit-for-bit reproducible. Some practical reasons why it wouldn't be are that various dependencies are of slightly different versions (e.g., one distro manages to get a new libc uploaded a little bit before another), that downstream conventions are different in different distros (e.g., Red Hat-style distros use /lib64 and Debian-style distros use /lib/x86_64-linux-gnu), or that a dependency has some shortcoming which many but not all distros patch in the same way, and the patch impacts things that use the dependency (e.g., upstream OpenSSL <1.1 does not have symbol versions, but most Linux distros patch them in). Yes, in an ideal world, all these things would be resolved, but there are going to be so many tiny things like this that come up that having infrastructure to accommodate them is the right plan.
I have seen many times Debian packagers try to build something from upstream and find that it just does not build anywhere other than the maintainer's computer. The fact that Debian requires that every package it ships can be built buy anyone in a generic environment is immensely valuable to free software, even if nobody used the binaries that Debian built. (And to be clear, other distros do the same, I'm just most familiar with Debian.)
I'd agree that in an ideal world, all the patches would be upstream and the binary would not just be equivalent but bit-for-bit reproducible. Some practical reasons why it wouldn't be are that various dependencies are of slightly different versions (e.g., one distro manages to get a new libc uploaded a little bit before another), that downstream conventions are different in different distros (e.g., Red Hat-style distros use /lib64 and Debian-style distros use /lib/x86_64-linux-gnu), or that a dependency has some shortcoming which many but not all distros patch in the same way, and the patch impacts things that use the dependency (e.g., upstream OpenSSL <1.1 does not have symbol versions, but most Linux distros patch them in). Yes, in an ideal world, all these things would be resolved, but there are going to be so many tiny things like this that come up that having infrastructure to accommodate them is the right plan.