It makes it much easier for users to retro-fit napari plugins to existing
it discourages people from accidentally duplicating info across both pyproject.toml and setup.cfg (if this happens pyproject.toml overwrites the information).
For an in the wild example of this problem, Draga and I got an email from some users confused about why their plugin was not being picked up by the napari hub. It turned out that they had classifier tags in both pyproject.toml AND setup..cfg, and so the napari framework tag was not being included in the packaged build (and therefore not available on PyPI for the napari-hub to discover it)
Closes https://github.com/napari/cookiecutter-napari-plugin/issues/167