That depends on which scenario. If you're parking a domain with a registrar, you're basically purchasing the domain, but you're not immediately pointing it anywhere. You might wish to do this if you're only planning to use the domain for e-mail, or if you don't have a hosting provider selected yet. Visitors that come to your domain will get a generic "This domain is parked with XXX"-site.
There's also something called "Parked Domains" in cPanel. This feature allows you to add a domain onto an existing hosting package, and basically redirect it to another domain. As an example, the domain "mighthost.net" is a domain that just redirects to our main domain. They're also known as aliases.