the traffic, promotion and seo efforts done on the second domain that is if it's added as parked domain via cpanel on the existing domain will yield new prospects or possibly sale depending on the amount of traffic.
Stole my thunder...its probably often to prevent the competitor from getting the name. The few names I've developed I've regged their cousins and pointed them to my name.
Thanks for the insight everyone. What is the best way to explain this to an end user who asks the question "why should I buy that domain, when I already have this one?"