Written communication is best for documenting something so that you can refer back to it. It can be used to protect yourself as well. Written communication is also used to reduce confusion and not having to rely on memory. Oral communication is best used for casual conversation and conveying of relatively unimportant information. In oral communication there is a high probability of misunderstanding and confusion, since people often hear the message differently than it was intended by the speaker.
The written communication should be used where no facial expression and bodily movement is required during conveying your message. The written communication may be within and outside the organization. In written communication we use letters, memo etc. While as in oral communication we don't use letters, memos etc. We only use tongue, bodily movements, gestures, postures to communicate our message. The written communication is important when we don't want to make face to face discussion.