Many domain owners decide to park their domains, because they haven't had the time to develop the domain name, yet. You must know that some domain owners hold portfolios with thousands of domains, so they cannot develop every one of them.
As for the traffic, parking companies and advertisers prefer type-in traffic. This type of traffic (also called direct navigation traffic) seems to be most valuable as it is well-converting. But parked domains sometimes receive link and search engine traffic, too, if they were developed before, for example.
These are the results from a web survey of .mobi that I ran about two weeks ago:
Number of .mobi domains in survey: 927369
Number of .mobi websites responding: 683584
type ¦ sites ¦
+---------+---------+
¦ F ¦ 3754 ¦ Forbidden/payment required
¦ X ¦ 4042 ¦ Porn
¦ S ¦ 5434 ¦ For Sale
¦ N ¦ 6329 ¦ Networks of sites (duplicate content)
¦ U ¦ 6352 ¦ Unavailable
¦ B ¦ 11274 ¦ Brand protection registrations
¦ D ¦ 15295 ¦ Dead sites/domains
¦ R ¦ 58933 ¦ Redirects (not classified yet)
¦ H ¦ 59272 ¦ Holding Pages (coming soon etc)
¦ A ¦ 69841 ¦ Active (HTTP 200)
¦ P ¦ 443068 ¦ Parked/PPC
The Active count and the Redirect counts will change as the data is processed.
A Brand Protection registration is often the .mobi/mobile web version of an existing site. Some of the redirects are redirects to the mobile web version of a site.
Regards...jmcc