Domain Name Registrars require the use of name servers or hosts for every domain registered.

In other words, every domain name has to be linked to a name server for it to be valid.

Some people do not have they own name servers. Not all Registrars offer 'domain parking' (name parking) on their servers. Domain Parking services provide this service. It is a convenient way to hold or 'park' domain name(s) for an extended amount of time.

And it is also a good way to make money, or monetize, your domain name.