Any money from expiring domains?

Why do people with domain names that have so much traffic, allow them to expire and be sold in the open market for just US$ 5?.

I've been trying to figure this out, as on the internet, if you have traffic - i.e. many visitors to your site every month, why would you want to give up the site name?. Could it be the owner has forgotten to renew?. Quite unlikely as the domain register will be contacting you way before the expiry date.

Some internet gurus say you can earn money by buying these expiring domains, and earning money from the current traffic clicking on the advertising links provided by the free hosting service they provide, and by reselling these domain names later.

Seems quite unlikely to me, but then again, since I have no experience, I can only make a lay man's guess. Anyone who has earned money from expiring domains want to share your experience?.