When you purchase a domain name, you are asked if you want to pay extra money for privacy, although, some domain registrars are offering it for free now. The privacy, or WHOIS protection, keeps your personal information, including name, address, email and physical address, out of the public WHOIS database. Major corporations don't bother with domain privacy as their public information is easy to find. If you were looking for Amazon's address, you wouldn't go to the WHOIS database first.
Individuals who don't opt for privacy often find themselves with a continuous stream of spam emails from companies offing SEO and other services that webmaster might need. Changing a domain to private after it was private won't necessarily help; the WHOIS database is scraped and the lists sold, so the person's information is already out there.
Despite the spam, there are times that having the webmasters information public is a good idea. A person who owns a small e-commerce business should already have the company's information on the website, but there are people who will check the WHOIS database anyway before spending several hundred dollars with a new business. A business that uses only a contact form with no address or phone number on the site, plus private WHOIS information, looks very shady.
A person who has a blog and talks about their own controversial opinions, should have their information private. A fashion or beauty blogger should keep her information private. Too many nuts and stalkers out there would think nothing of visiting a blog owner's home if it is within driving distance.
Most of us would be upset if a company we do business with accidentally posted our name, address and phone number online, but when a person buys a domain and does not opt for privacy, that is exactly what happens.
Not all top-level domains are eligible for privacy on the WHOIS database. The .us and many other ccTLDs are not eligible for privacy. For more information click here https://www.reddit.com/r/web_design/comments/8tbsm4/designed_a_website_didnt_pay_initially_for/.