It would be useful to compare with "eslint --fix" given Eslint can already reformat (on top of linting non-style things, and projects usually already have Eslint settings).
Unlike ESLint ... everything is fixable. This works because Prettier never "checks" anything; it ... parses your JavaScript into an AST (Abstract Syntax Tree) and pretty-prints the AST, completely ignoring any of the original formatting.
There's an extremely important piece missing from existing styling tools: the maximum line length ...
(ESLint never knows how to fix it).