With the spoken language, there are subtleties that underly the words such as voice inflection and facial expressions.
The written language needs to have these subtleties clearly identified within the description otherwise the true meaning might be missed. This is especially true for sarcastic remarks.
The benefit of written language is that is as permanent as the medium that it is written on. That is why we know what the ancient civilizations were saying on their stone tablets. The spoken language is temporary and ethereal and only benefits those who were able to hear it.
The differences between spoken and written languages is seen in terms of the transfer method of information. Spoken language involves dynamism of oral communication, while written language is fairly simple and static. Spoken language is full of repetition, while written is more formal. Find out more from http://www.omniglot.com/writing/writingvspeech.htm.