Devices like iPods, iPads, and computers that support iMessage (the instant messenger thing, so to speak) do not have a phone number. So, if you are trying to message someone who doesn't have a device with a phone number, you can put their email address, instead of their phone number, in the "To" section when writing a message. The email address serves as a phone number for devices that don't have a phone number.
Additionally, having an email address alternative for your iPhone is pretty useful. If you are sending messages to a friend in another country, you should create an iMessage conversation with their email address instead of their phone number. The reason for this is because sometimes messages get sent as texts if they fail to deliver, and it could cost you a lot with international rates.
Additionally, having an email address alternative for your iPhone is pretty useful. If you are sending messages to a friend in another country, you should create an iMessage conversation with their email address instead of their phone number. The reason for this is because sometimes messages get sent as texts if they fail to deliver, and it could cost you a lot with international rates.