Specialty leasing companies?

[paradocs98]

Bit of a random question, but something I've been wondering for some time:

I've noticed in this area (Long Island, NY) new, luxury cars with dealer license plate frames or dealer decals that say "Power Motorcars" or "Benjamin Motors." I believe that both are just leasing companies that get ahold of new cars and lease them to customers. I know that neither is an authorized BMW/Mercedes/Porsche/etc. dealer in the area, and yet I see this identification on a lot of new cars around here. The cars seem much too new to be used/CPO.

So what's the advantage of going with one of these third-party companies? Do they get much better lease rates for their customers? Any downsides to not buying/leasing from an authorized dealer?