Domain name registration requires sharing the Registrant’s information, in whole or in part, with the applicable Registry Operator and with ICANN. As required by ICANN, this information must be made publicly available by means of Whois, Registry Operator also may be required to make this information publicly available by Whois.
When you purchase a domain, your registrar is required by ICANN to enter the contact information of the site owner. Some kind of information must be submitted. So unless you opt to protect your personal data, it goes public.
Privacy protection third party paid service hides personal contact information from complete strangers. This is pretty straightforward and is one of the most common reasons domain owners opt for privacy protection.
Domain privacy or WHOIS protection (different registrars sometimes refer to it by different names) hides the domain registrant’s personal information from the public WHOIS database.
If someone knows a registered domain name, they can enter it into any WHOIS search tool on the web to retrieve this public information. With privacy protection in place, all of this information becomes masked.
for more details : https://www.icann.org/resources/pages/responsibilities-2014-03-14-en