From what I understand and to continue your example of Ubuntu-based distros:

As you say, Ubuntu itself is corporate-driven, so there are things in there that exist pretty much solely to benefit Canonical (e.g the telemetry they recently introduced if i recall correctly)

Most of the time when basing distros off of others, I think it's to keep a lot of features - either to save dev time or because they only want to tweak a small portion of the distro and not write a new one from scratch.

Because devs can modify the entire codebase, they can remove features that are corporate-driven (telemetry and such) and effectively create something fully (or mostly) compatible yet without such features.

Another major example imo is the removal of snaps, which most people (myself included) strongly dislike - as far as I'm aware removing them in Ubuntu itself is quite a difficult process as it's baked into the distro itself. I imagine a lot of people want something like Ubuntu as it is quite friendly and has one of the lower bars of entry for Linux, but object to corporate things like telemetry and the overall monstrosity that is snaps.

Apologies, i went down a bit of a tangent, but I hope that roughly answers your question!