While writing may be the preferred mode for a contract, in many other contexts, speech will be more appropriate.
Because the most common type of speech—face-to-face conversation—is highly interactive.
This mode is well-suited to many social contexts, such as casual conversations over lunch, business transactions in a grocery store, discussions between students and teachers in a classroom.
And in these contexts, interactive dialogues have many advantages over writing.
For instance, individuals engaged in conversation can ask for immediate clarification if there is a question about something said;
in a letter to a friend, in contrast, such immediacy is lacking.
When speaking to one another, speakers are face to face and can therefore see how individuals react to what is said.
On the other hand, writing creates distance between writer and reader, preventing the writer from getting any immediate reaction from the reader.
Speech is oral, thus making it possible to use intonation to emphasize words or phrased and express emotion.
Of course, one might say that writing has punctuation.
Well, it can express only a small proportion of the features that intonation has.
Because speech is created "on-line," it is produced quickly and easily.
This may result in many "ungrammatical" constructions, but rarely do these rough sentences cause miscommunications.
You know, if there is a misunderstanding, it can be easily corrected.
On the contrary, writing is much more deliberate.
It require planning, editing and thus taking much more time to produce on the part of the writer.
Because of all these characteristics of writing, if an individual desires a casual, intimate encounter with a friend, he or she is more likely to meet personally than write a letter.
In this case, writing a letter to a friend might turn out to be too formal.
Of course, in today's world, the highly-developed technology has made such encounters possible with "instant messaging" over a computer or a smart phone.
And if someone wishes to have such an encounter with a friend living many miles away, then this kind of on-line written "chat" can mimic a face-to-face conversation.
But because such conversations are a hybrid of speech and writing, they still lack the intimacy and immediacy of a face-to-face conversation.