The benefit is they either couple or decouple a circuit, they either filter or store energy. You need to place a cap at the point where you want its action to take place. How many farads depends on how you're using the cap. If you are using it as a storage device, get as many as you can.
But I bet you meant to place this in the "car stereo" section.
Yah, well i put it in subwoofers because there is far more traffic here than the other places. I guess I dont need a capacitor, but i have heard of them and seen several. Just never knew if one would be of any use to me.