While the job description and your subsequent application will have assured hiring managers that you have the necessary qualifications for the role, an important part of the recruitment process is ensuring that you'll be a good fit for the company culture.
The best candidate for a job will be the person with not only the essential technical skills, but someone whose positive attitude and soft skills will enable them to work effectively with future colleagues.
The job interviewer asks you to tell them about yourself in order to gain insight into you as a person. They will be observing your personality traits and your confidence as you are talking.