There are many domain name registration companies that have been accredited by ICANN. What is the purpose why they are accredited by ICANN?
They are a non-profit organization that oversees all aspects of internet life. They allow these registrars to register domains to their clients. It is like a regulatory company.
If you want to get an accreditation there you need to have several steps completed. Domain name registration is a business and ICANN is regulator which delegates rights for domain name registration.