

One of the few mail providers to integrate OpenPGP and also support custom domains.
You can have them encrypt your mail with your public key as soon as it arrives, so that your mailbox is always encrypted.
If you trust them to store your private key, you can use their web interface to decrypt your messages in the browser. Or you can have complete custody of your own keys and use your own external mail client.
You can also turn off mailbox encryption (the default) and just use it as a regular mail provider.
Yes, but this is true of all standard email services. If you want end-to-end encryption then you need to have your correspondent encrypt their message with your public key before sending, as with any E2EE email.
The purpose of encrypting incoming mail is for mail that arrives unencrypted (most mail) so it does not sit in plaintext on their servers.