Endings

A lot of single player games have multiple endings. Some of them also require multiple new runs to obtain all of them. Some are also way more punishing than others. Undertale is one such game. If in your first playthrough you decided to just kill everything and do the genocide run, you would've ruined any chances of ever doing a true pacifist (not kill anything with additional requirements to get the best ending possible) run without quite a bit of hassle.

Undertale is known for fourth wall breaks where things such as reloading an older save doesn't completely undo everything you did.

Restarting the game after your genocide run, you will be faced a black screen for 10 minutes until given the choice to sell your soul to reset the world you destroyed.

The game would then literally save that fact to your steam cloud.

It you then attempted a true pacifist run, you would get a twisted ending.

Even if you deleted the file, everytime you'd restart the game, Steam would just redownload it. So now you're left with the option of never being able to play the game online again (and making it impossible to gain any further achievements) or to follow some random guide on the internet to find out how to restrict file reading access for that specific file.