There are some obvious benefits to using oral communication over written communication:
Tone: It is possible for tone to be misinterpreted in the written word, and for readers to infer things that were perhaps were not intended.
Nuances of language, such as sarcasm, are often lost in the written word as in every day life they are usually picked up from the tone of a persons voice or facial expression.
Interactivity: I think the main benefit of delivering a message orally is the opportunity to create a two way dialogue with the person. You can react and respond to their reactions, eg. Body language and facial expressions.
If they appear confused, you can explain a particular point in more detail. It also gives them an opportunity to respond with questions that you can answer. A written document is unchangeable, and cannot be expanded upon.
The personal touch: People often prefer for messages to be delivered orally if they are of a sensitive nature, and find a letter or email impersonal and cold.