To create money making opportunities for language schools and websites like this.

Early humans could neither speak nor write, they communicated with each other by using body language and making ape-like grunting noises.  Then, some favorable genetic mutations occured in humans about 100,000 years ago in Africa that gave these humans the ability to speak.  Speech allowed them to better communicate with each other, and they were better able to teach each other how to make better tools for hunting and making fires.  These newly acquired skills enabled humans to migrate out of Africa and spread throughout the world.  As groups of people migrated throughout the world and they became more isolated from one another, each population's language evolved differently from the original common language of humans, which existed 100,000 years ago.  Different accents exist within each language because of the educational and social class differences between individuals within a society. 

Do you mean to ask "Why are there different languages and different accents?"