Wheel spacers are used for a number of reasons, from form to function. For cosmetic reasons, you may want to fit wheels with an offset that isn't designed for your car or simply to help bring the wheel face out to be more in line with the arches. For the function side, wheel spacers are used for increasing wheel clearance for big brake calipers or simply to improve the handling by increasing the track of the car.
Hubcentric wheel spacers are designed to make sure the weight of the car is carried by the hub and not by the studs or bolts. They are machined perfectly on the rear of the wheel spacer to match the locating ring on the hub with the same size locating ring on the front of the spacer. This means that the wheels is truly central to the cars hub, meaning no movement and no vibration through the wheels. A hubcentric wheel spacer also result in far less stress on your bolts or studs.
Non-Hubcentric Wheel Spacers
However, with a non-hubcentric wheel spacer, the weight of the car is no longer on the locating ring and is now on the studs, this will lead to vibrations from the wheels.
Hubcentric Wheel Spacers
As you can see in the cross-section diagram of a hubcentric wheel spacer above, the locating ring fits into the space on the wheel exactly as the hub's locating ring would. This means that the weight of the car is still resting on the hub rather than the wheel studs, meaning no vibrations from the wheels.