With e-mail you can save a record of your conversations with other people and be able to verify exactly what a particular person wrote on a particular date. At MFM we archive our mailing lists so subscribers can browse past messages and search them for keywords. This is enormously useful for finding solutions to technical support problems, for example. This is useful whenever a group of people wish to invite new people to join an ongoing conversation. With e-mail you can simply forward copies of the old messages or reference them in an archive to permit the new participants to catch up.
Most people comprehend technical information better by reading it than by hearing it.
Complex procedures, specifications, computer commands all travel better in written form. This is often much easier than reciting complex commands full of obscure characters over the telephone. Once a group of people have structured their work habits around e-mail, the members of that group can work anywhere in the world that they desire. There are other advantages of e-mail that become obvious as you use it. As you read hundreds of e-mail messages from dozens of people, you'll discover that some people are better than others at creating messages that are easy to understand.