Although basic communication is possible without words, there are certain situations in which verbal language is essential. As soon as speakers refer to matters that are beyond the 'here' and 'now', gestures alone do not work. For example when we talk about the past or the future, when we refer to abstract matters such as speed, happiness or politics, or when we make a joke, it becomes very difficult to convey those meanings through body language.
How do you point at something that happened yesterday, or will happen tomorrow? How should you read a smile when you cannot see what people are smiling about? The missing link in all those situations is language.