@AugierLe42e @toast @bitecode It's explained in great detail in that link.

I was not involved in PEP 517/518, but as a maintainer of setuptools trying to migrate people to setup.cfg, I am constantly annoyed by INI files.

There are tons of upsides to putting this entirely new thing (it doesn't replace anything that has ever existed in setup.cfg) in a TOML file. The only downside is "some people might need to add a new file to their projects or rename an existing file." That is not a high price to pay.