For example, it's trivial from a web browser with a couple of clicks to go and find out all the downstream changes to a package. For example to see how glibc is currently customized in debian testing/unstable you can just navigate this webpage:
https://sources.debian.org/src/glibc/2.42-6/debian/patches
If everything gets merged in the same git tree it's way harder. Harder but doable with a rebase+force push workflow, which makes collaboration way harder. Just impossible with a merge workflow.
As an upstream maintainer of several project, being able to tell at a glance and with a few clicks how one of my projects is patched in a distribution is immensely useful when bug reports are opened.
In a past job it also literally saved a ton of money because we could show legal how various upstreams were customized by providing the content of a few .debian.tar.gz tarballs with a few small, detached patches that could be analyzed, instead of massive upstream trees that would take orders of magnitude more time to go through.