AFAIK, PGP is only automatically used in emails to other Proton users, you need to do it manually if you want to communicate with someone else with PGP (or use the secure email thing, which does it on Proton's servers). So the PGP is largely just an implementation detail in how they store it, unless you're communicating with a lot of other Proton users.

Then again, it's been a couple years since I used Proton, so I don't know if things have changed. But since nobody I contact uses Proton or Tuta, it's irrelevant that Proton uses PGP. If I use PGP, I'd do it myself regardless.