A linked email lets you recover your account and purchases on a new phone, even if you lose your recovery key.
Open Settings → Account and tap the edit button next to Recovery email. Enter your address and tap Send code, then enter the 6-digit code we email you. The code is valid for 10 minutes.
An email address can be linked to one account. We use it only for your account, never for marketing. You can unlink it in the same place.