"Posi-traction" is GMs word for "Limited-slip"....same thing. Other companys had to come-up with different names because of trade-marks.
"Posi" is a "clutch-type" differential. That means that the rear wheels are still allowed to run at different speeds during a turn. (The wheel on the outside of a turn has to travel further/faster than the inside) The "clutches" are spring loaded and apply some force/resistance to this faster versus slower action. This keeps the wheels more balanced as far as which gets more power, yet still allows for slippage to keep the differential action.
"Open" differentials do not have the "clutches" that try to keep things equal, so which ever tire gets the least traction...that is where the power goes.
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( when i gas hard while turning, one wheel spins)
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This would be the tire on the inside of the turn. The body rolls to the outside and removes some of the weight from the inner tire....less traction, it spins and the whole truck slows down because the power is spinning one wheel, not pushing the truck forward.
"Lockers" are different yet. They do not "slip" they go back and forth between locked solidly (no differential) and unlocked where they act more like an open differential.
There are several variations of most of these differentials, this is a very simple explanation....but I hope you get the idea.
As far as whether you "need" a Posi or not............If you didn't know that it wasn't there, you don't "need" it. A Posi will make your truck act/drive differently in "lower traction" situations. Snow, ice, and rain will cause traction problems.......even though it sounds backward, a Posi unit makes the truck more unstable in bad weather. Say you go around a sharp turn in the rain, "gas it" like before....instead of the one tire spinning and the truck following the turn like you expect...
both tire spin and the rear of the truck slides toward the outside of the turn.
I'm not saying that a Posi is inherently dangerous, but you do need to drive it accordingly...and so does anyone that you let borrow it.