These days if you register a domain with a registrar but don't develop a webpage on the site it becomes what is known as a "parked page." Visitors to these sites don't get a 404 error or that old "under construction" sign but instead ads related to the URL and provided by Google, Yahoo and others.
For me, these pages represent the beautiful, entrepreneurial, random nature of life, because you just know that many were registered with the hopes of building a website or business but the owner never got around to it. Maybe they realized that OneHourSofas.com was just a bad business plan or they broke up with their girlfriend before Jennyisawesome.com could be finished.
And let's face it, i'm no different - I hold (or have held) a bunch of random domain names after fleeting moments of inspiration. This explains my recent pursuit and registration of Staycation.com after many months of stalking. For the uneducated, staycation is a phrase of limited popularity used to describe the act of taking time off but hanging around your home/home city in order to take care of stuff that you never do while working.
What do i want to do with Staycation.com? I have no idea - likely nothing. But just as the cost of a lottery ticket gives you license to dream about the jackpot, owning a domain name is a relatively cheap option value on the idea of building something to share with the world. And at <$10/yr, who can argue with that?