Spacers have long been used in skateboard wheels to set you bearing in the wheel correctly (see last photo for where it sits inside a wheel). Personally, I find them to be somewhat an unnecessary in standard skateboard wheels, as most modern wheels will seat your bearing fine without the need for them.

In saying that, if you are running a duel duro wheel, cored wheel, or are running them on a longboard or downhill bomber, then they do start to serve a purpose. When wheels hit higher speeds, the bearing can begin to get hot. This heat expands the bearing & can put more friction heat into the wheel, the bearing spacer helps keep the bearing in the correct placement even with the deformation on the wheel & bearing housing.

In short, for general park & street skating, you probably won't notice if you don't have these, but if you're putting the wheel in some serious speed situations or long held side pressures, use these then

Spec's

Quantity: Single - OR - Full Set of 4
Size:
   Across Axle Width - 10mm (Standard wheel core size for all good quality wheels)
   Internal Diameter - 8mm (Standard skateboard axle size)
   Exterior Diameter - 10.5mm (Fits all good quality wheels)