Email is particularly convenient because it is asynchronous; it waits for you and fits into your schedule instead of demanding that you structure your activities to synchronize with those you communicate with.
For example, an email recipient doesn’t have to be available when you compose and send an email — you can send it at the time that is most convenient to you. Similarly, you don’t have to be available or even connected to the Internet when someone else sends you email — it waits on your server until you log in and download it when most convenient to you.
Email provides the convenience that voice mail later provided for telephones — except that voice mail is more ephemeral, cannot be conveniently edited, and is usually accompanied by a preference to talk to the other party in real time. With email you know that the medium is inherently asynchronous, so you tend to write down all the information the addressee needs so they can respond when they are able.