The default answer to a congestion question is a single average: average arrivals, average service time, average wait. That answer is unsatisfying because it cannot say how close the system is to tipping, how variable the wait becomes near the limit, or what happens once demand exceeds capacity. What the Jev model promises is a description of the whole operating range, so the calm regime, the approach to the limit, and the overloaded regime are all visible in one picture.
The Jev Model: An Intuitive Overview