An oral direction, of whatever nature it may be —it could be the instructions to get to a place, or how to accomplish a desired objective, for example— should be memorized in order to not forget its details and get lost in the way of accomplishing what you wish to do. However, since memory is often deceptive or flawed, it is always wise to write down the directions so as to not depend only on memory.
True because they are being spoken to you, if you don't remember them or write them down you don't have them therefore you will not know what you are doing.
