There is a popular belief, generally put forward by human resources departments, time management consultants, and other "experts", that formal communication and discussion between people in the workplace should never be handled by email (i.e. written). "Important" communication should be handled by voice.
Preferably, they say, this communication should be in a face to face meeting. If that's not possible, they recommend using the telephone. The rationale that is typically given is that written methods of communication (i.e. email) don't convey tone and facial expression.
A recently published study would seem to support this view.