you want a quick but not an instant response email will get to the recipient more quickly than a letter or a video but you won't get a response as fast as with a phone call or turning up in person
your audience is at distance the email is an online medium means that distance is on object it will be much cheaper than a long distance call and faster than the snail mail
your want time to compose your messages use email in preference to a phone call or in person meeting if you need to compose your message carefully
you want time to compose your message whether it's stored on disk or printed out an email can give you a permanent record of your message something that's more difficult to achiere with the phone or working face to face