Domain Parking is big business.  When you click on a link somewhere or make a typo entering a web address and you wind up on some crapola "search page" optionally throwing up a million pop-up ads? That is a parked domain and the larger players can park thousands of domains and make literally millions of dollars "monetizing" them via domain parking.

Who has access to thousands of domains? Domain registrars. Some of them offer domain registrations and rock bottom prices just so they can monetize the parked names. This may not bother you, but some people don't realize they're paying for something their registrar then uses to generate more revenue for themselves.