Fast and cheap
Ideal way to reach persons who are difficult to get on the phone (e.g. most physicians!)
Because it is written, you can compose and review your message before sending
Ideal for international communications (expense, problems reaching colleagues, differences in time zones)
Convenient and non-intrusive: I typically review and answer emails late at night after the kids have gone to bed
You can check your email from any computer in the world; e.g., while visiting, from a hotel room, in your car, etc.