Some things are just designed for abuse.https://www.techrepublic.com/article/google-mozilla-working-on-letting-web-apps-edit-files-despite-warning-it-could-be-abused-in-terrible
The obvious application for saving files though would be things like keys for UIs like RiotWeb. Saving keys in the cache isn't a good idea.
If it was restricted to that type of use I'd be ok with adding the ability to access the filesystem. Anything much beyond that just becomes an attack vector.