The BBQ Pit was originally envisioned as a place where folks could go to flame someone (hence the forum name) without repercussions from the moderation staff. When you have forums that don’t have anything like a Pit, when people get angry and don’t have an outlet for it, that anger tends to build until they can’t contain it any more. Then you end up with a mess in your forum from someone blowing a gasket.
The idea of the Pit is that you can flame someone to your heart’s content. That gets it out of your system and keeps that anger out of the regular forums.
The Pit quickly expanded to flaming pretty much anything that was bothering you, instead of just being limited to flaming someone here or something that they did. The rules-free environment also has a certain appeal to some, so the Pit has definitely grown from its original intention.
The key here is that it is generally rule-free. That means you don’t have to worry about picking up a warning for what you post there (mostly - there are a few rules).
There is no protection against your reputation, though. Anything you post in the Pit is there forever, for anyone to read. A lot of folks here don’t read the Pit, but many do, and they can always link to your Pit thread even in non-Pit posts.
You can be an asshole in the Pit and not get moderated for it, but that won’t prevent others on the SDMB from thinking that you’re an asshole.