Domain registrars most of the times charge a hefty fee for complementary services such as WhoIs privacy which is used for masking public details of your domain name such as your name and address. In some cases the cost of WhoIs privacy service is more than the cost of the domain itself.
Truly speaking it is a very profitable venture for domain registrars. But this entire model got disrupted recently when EU (European Union) decided to go ahead with GDPR implementation in which it is illegal to publicly show someone’s personal information. This is particularly applicable for EU residents but it has made its impact beyond that. Registrars are now forced to show truncated version of whois to the public. So for end users like us it is a sort of free privacy forever.