Axle washers (AKA- Speed Rings) reduce the friction the inner race of the bearing gets when it rotates while with pressure against the hanger of the truck or axle nut. This part, unlike wheel spacers, we belive is a needed part for any skateboard!

Here's the reason!
Before we get into it, you need to understand the build of a bearing.
All modern bearings are made up of 4 to 5 parts;
1- Outer Race (larger outer ring on a bearing, holds it in the wheel & guides the ball bearings in a channel on the inside of it)
2- Inner Race (the smaller ring that makes contact with the axle of the truck & guides the ball bearings in a channel on the inside of it)
3- Ball Bearings (7 in standard bearings, 6 in bigballs. Little spheres of a metal compound which rotate inside the races allowing the bearing itself to rotate)
4- Crown (this is what keeps the bearings evenly spaced apart & stops the bearings from chattering together & falling out)
5- Shield(s) either on both or on one side of the bearing to stop dust, etc. from getting into the bearing.

When you press a bearing into any good quality wheel, the outer race makes contact & seals against the wheel & will rotate as if it's part of the wheel, while the inner race rotates against the truck axle but can spin even though the axle itself doesn't rotate. The ball bearings inside the wheel do the most rotation but the inner race will still rotate.
With this in mind, when turning etc. on a skateboard, the inner race of the bearing will make contact with both the truck hanger where the axle protrudes & the axle nut holding the wheel on (this is also why when the Nyloc in an axle nut fails the nut will unscrew itself).
By placing a washer either side of the wheel, the bearing will make contact with another rotatable surface, reducing friction & allowing the bearing to spin at a high speed even when it takes on side pressures when turning. 

Does it really make that much of a difference though?
It will extend the life of the bearing by reducing the fatigue it takes from side pressures, I find my wheels spin faster & longer then mates who don't bother, & have gotten years out of bearings (currently have a set of bearings 20years old in one set-up, replaced the crowns in them too just cause I wanna see how long they last).

The other upside is truck axles are designed to run them, if you run a normal wheel & bearing combo with a speed ring either side, the axle nut sites flush with a small amount of "wiggle" room for the wheel to move slightly either way on the axle (a little wheel rattle means the wheel is spinning freely & also can add to the sound of a revert).

You can take this one step further & run 3 washers per wheel; 1 washer axle side & 2 hanger side, pushing the wheel out slightly more, allowing alittle less "wiggle' room & making most trucks axle nuts site perfectly flush reducing axle nuts failing when removing due to cross-threading. Best done on brand new trucks that don't have a warn axle end

Spec's

Quantity: Single - Full Set of 8 (2 for each wheel) - 3x Per/Wheel (Set of 12)
Size:
   Inner Diameter: 8mm
   Outer Diameter: 11mm
   Width: 1mm