You can just install all game stores on Windows and there is no downside to doing so, and no real difference between installing the Epic store and purchasing a game there and buying the game on the game publisher's website.
Also currently Steam has a dominant position and 30% cut, and if the Epic store succeeds it will result in a more competitive situation with at least two game stores probably taking a 5-15% cut, which is a drastic improvement for everyone except for Valve, which makes Steam.
Same thing on mobile if Epic succeeds in the antitrust lawsuit with Google/Apple stores instead of Steam.