I see that most domain names that contain some nice words like: www.pixelmania.com , www.musicbox.com and so on... are registered but look just auto-populated with some random data. Why they do this ? Is it for me to pay extra to get that domain? Is it for advertising purposes (all of them have insane amounts of ads on them)? Or what ... ?
Its nothing. Just few bunch of selfish freaks, who are planning to make more money selling them in future.
But sometimes... they are also used to reserve the domain for future development to avoid the risk of losing them in future. And they try to utilize this unused to domain by advertising and earn a little money on the mean time. :D
There are several reasons
for money - sell the name for more;
future expand of the original site (local site for every country);
expired domains as John Conde sayd. I think the ads and the random data is available only in the "possible renew" period;
there is a small number of domains that are registered for system use only (.arpa). Not Exactly sure about "net.net" ...