why does it need email?

Because you can send sats to an email address which has Proton Wallet enabled. No address reuse, gpg signed btc address by your gpg private key but probably exposing the bitcoin addresses or in the worst case even the xpub to @Proton.

Proton

Guide to using Bitcoin via Email | Proton

Send and receive bitcoin easily and privately with just an email address using Bitcoin via Email, a new feature in Proton Wallet.