WhoisGuard is a service connected to your domain if purchased during the registration, renewal and later on. WhoisGuard is a privacy protection service that keeps the privacy invaders from seeing your name, phone number, home/work address, email address, and more information. WhoisGuard replaces your information with theirs to protect the users from identity theft and potential spam.
During the purchase of the domain, the registrar asks the users to fill up the information such as owner name of the domain, home/work, personal phone number to contact, telephone number, email address, and other information.
There are thousands of free online tools available for free to use, and anyone can view the information without your knowledge. Such domains face a potential threat from spammers, privacy invaders, hackers, and domain stealers.
Example: WhoisGuard Protected
The above snapshot explains that the domain owner and details are replaced with GoDaddy registrar. In case, if the domain is violating policies of registrar, then you can email them to take necessary measures.
Those who did not purchase the extra protection suffer from visible information in public.