1. When the .Com version of the same (generic) domain is parked. 2. When the domain itself is just a really nice domain.
For example, when it's a domain like:
(State)Attorney.net
(State)Insurance.net
(State Abbreviation)Attorney.net
(State Abbreviation)Insurance.net
What's nice about having the parked .Com domain is that many folks will default to checking the .Net.
What's nice about certain .Net domains is that they are 1) simply so robust in their generic quality; and, 2) their converted lead and/or PPC value is SO high that only a few visits will be all it takes to make the investment pay off.
So, anyone have another reason why or when it's nice to be a .Net? :)