FWIW, here is a use case that I have. I’ll be concrete, but hopefully not so detailed as to be distracting.
The xarray package has two dependencies: numpy and pandas. My package uses numpy, and could use some features of xarray that don’t require pandas, but I don’t want to pull pandas into my dependency tree. (I’m willing to accept ImportErrors if my code accesses pandas-dependent features.)
As it stands, I would either need to convince them to break out the pandas-independent concepts into a separate package (plausible, but a nontrivial effort), or move the pandas dependency into an extra and make all packages depending on that functionality install xarray[pandas] (non-starter).
If I could submit a PR to make xarray[minimal] or xarray[!pandas] a way to just get xarray and numpy, the maintenance burden on xarray would be substantially lower. It would also only (okay, mostly) be used by downstream packages like mine, that understand the risk they’re taking to not pull in all dependencies and to watch upstream to ensure that functionality is maintained with the reduced dependency set.