Here is the thread on what a catch can is:
Billet Technology was the first to develop a catch can for the current Generation HEMIs. All of the other companies out there have either tried to or have copied their design. What it does in a nutshell is it collects the oil vapor coming from the PCV valve back into the intake manifold.
When I had my 2011 RAM, I had one with the extension (that they do not sell anymore) and I had to empty it out about every month or so. On my '06 SRT Charger, I'm dumping it about once every 2 weeks. Plus when I first cleaned my TB, I notice about 2-3 inches of oil inside my intake manifold that came from the PCV valve and it gets changed when it's required by the maintenance schedule.
I had one in my Ram, Durango, and my SRT and I am planning on installing one in my new 3500 as well.