You might have an iCloud account you don’t use for Apple ID and syncing purposes, but just email.
It’s easy to wind up with multiple iCloud accounts. You could wind up using different accounts for different purposes or sets of machines and want to consolidate receiving email for all of them. Apple doesn’t offer an obvious way to add iCloud accounts just for email in the Mail app. (Any third-party app makes it easy.) That’s because Apple steers you to select iCloud as your account type in the setup process, and only then warns you that account would then be used for all your iCloud purposes, not just email.
There’s a way to do it that requires using an app-specific password, which is Apple’s term for a password used exclusively for email, calendar, and contacts access. This kind of password is required because all other syncing is locked inside iCloud.com’s secure and proprietary services. Email, calendar, and contacts rely on industry standards, and it’s common to use third-party apps with them.
