Handle multiple NON-Protonmail emails from Protonmail
GMail allows to handle accounts from other providers all from within GMail. Say I have another account on Hotmail: I can register that in my GMail account as well, and then I am able to receive and send emails as if I were doing so from the Hotmail account but from the GMail interface. This is not like forwarding from Hotmail to GMail: I can also send emails coming from my Hotmail account but within GMail.
This is really useful if one has multiple accounts from different providers.