Most of us intuitively understand that there are differences between oral and written language. All communication includes the transfer of information from one person to another, and while the transfer of information is only the first step in the process of understanding a complex phenomenon, it is an important first step.
Writing is a fairly static form of transfer.
Speaking is a dynamic transfer of information. To be an effective speaker, you must exploit the dynamism of oral communication, but also learn to work within its limitations. The written language can be significantly more precise. Written words can be chosen with greater deliberation and thought, and a written argument can be extraordinarily sophisticated, intricate, and lengthy. These attributes of writing are possible because the pace of involvement is controlled by both the writer and the reader.