You can get down to the business of conveying information more quickly. The writer should have that purpose clearly in mind during the entire time the memo is written. Memos should exhibit clarity of thought and logic. After all, these are your colleagues. You've heard all of these things before.
People like to use e-mail because it is efficient and inexpensive.
E-mail is taking the place of messages on a telephone answering machine because of its convenience. Although thought to be more casual, e-mail in a business setting demands that writers apply certain rules of good communication. Don't assume that the recipient knows what you are talking about. Explain yourself fully but efficiently. More information is better than less.