Whenever you register a domain, your name, email, phone number and complete address is recorded in the public WHOIS database. The WHOIS was created to ensure online transparency; however, in recent years it's been used or harvested by spammers, hackers and identity thieves.
Though there are some exceptions, such as .BD domains, if you have not specifically enabled domain privacy protection, then your information is on the WHOIS for everyone to see through easily accessible online tools.
Quite important! Without it you will almost certainly be spammed by a variety of different services (website tools, ‘enlargement’ pills, weight loss programs, etc.) the ‘too-good-to-be-true’ emails (that prince from that foreign country just needs you to transfer money) and the random nonsense types (click here to unsubscribe!). Any of these could have viruses or programs to steal information off your device. Worse, hackers and identity thieves can potentially use the information to attack you or steal from you. Hence, opting out of privacy protection exposes you to greater risk.
Yes. Certain extensions are managed by registries that have chosen to opt out of the privacy protection service, or that are subject to certain requirements that WHC does not yet support.