They go between the engine the mount usually. They disipate vibration so the car dosn't shake itself stupid or break the engine mount itself. Poly ones are more rigid and don't allow the motor to torque over as much. Generally used in hi performance applications. That is if the motor isn't solid mounted. It sapossed to allow more power to be tranfered through the drive train out of the hole instead of the motor torquing to one side and then distibuting it's full potential outward. Also helps keep everything inline instead of twisting it with the force of the engine. If you look at big power Prostreet drag cars. Sometimes you'll see the passenger side wheel off the ground when they leave from the light. That's the engine torque pulling the frame. I've seen a couple instances where the winshield craks because of that. I had a budy that had an 11sec Grand National with T tops. the engine twisted the frame enough that the T tops didn't fit right and caused them to leak when it rained.
not unless you plan on doing some serious autoX/ road racing with it. If it's gona be a daily driver and you want to modify some stuff that $300 could go twards a lot more usefull items then engine mounts. Like stiffer springs, adjustable shocks, sway bars, engine mods. Good stuff like that. Besides, engine mounts aren't the most exsiting thing in the world to change. Especially on a new car with their oh so cramped engine compartments. And could potentially void the warrenty. Since the car is practacily brand freak'n new, I wouldn't worry about them so much. Honestley, I'm suprised they even make such a thing for that car already!