Ejectors are flow devices that provide low back-pressure to a duct by passing a gas jet past the duct and taking advantage of the low pressure region this creates (See figure). A common usage of this device is for paint spray guns. I outlined the contraction relevant to my question in red.
My understanding is that the static pressure of the jet is what provides the low back-pressure. If this is correct, I do not know why there would need to be a contraction/venturi assembly downstream of the suction manifold. A hypothesis is that because the jet is likely supersonic, this is needed to diffuse/slow down the discharge gases.
I would also appreciate any insights into what advantages/disadvantages an ejector has compared to the type of suction device used in carburetors. This comparison was actually the motivation for the question because an ejector looks very similar to a carburetor venturi.
Despite the above, I will specify my question: What function does the venturi like assembly in an ejector serve?