When registering a domain you must provide valid contact information which is accessible from the public Whois database of all domain registrations.
Data miners often crawl public Whois records for personal information leading to increased spam and junk mail. With Whois privacy protection your domain is registered with anonymous details to protect your personal information.
For example:
Whois privacy protection is now free for all eligible domains and enabled by default.
Whois privacy is not allowed for .us domains and can only be enabled for .ca domains when registered by an individual.