Polyfill is something that I've seen as an attempt to make a box seem bigger. If you can, I always suggest getting proper airspace with a proper enclosure; I understand that's not always doable.
Other than that, polyfill is used in things like midrange towers, where they are ported. The polyfill helps keep midrange reverberations from coming out of the port, because the polyfill absorbs or dampens the sharpness of higher frequencies, so that only bass/midbass frequencies come out, so it doesn't mess up your angles/staging of the midrange bandwidths. My 42" Klipsche towers have polyfill done this way, for example. They sound really good. Play down to 40-45 hz, and play up past 20 KHz, and have 2 backwards firing ports. The polyfill is layered on the inner walls, on every wall, so that the midrange bandwidth doesn't come out of the port and reverberate off the house walls- only the bass does, because it's more like omnidirectional pressure and not angular like higher frequencies are. I imagine if people have ported midranges in car doors or in home-like setups that using polyfill in those would really help not have unbalanced midrange sounds coming out of the port, at weird angles and weird peaks.