Why would I care if one is merely “formatting” or not? If I have to run a tool either way, I would prefer one that accepts a user-friendly input language and decouples content from presentation.
Because transforming an .md file into an .html file is a lot more invasive (though taken for granted here I think) than just writing the .html file. It's a build step where there wasn't one before.
I'm not saying it's never worth it.