The cheap ones act like a big washer spacing the wheels off the hub. they need longer bolts (not supplied) to lash it all together. when fitting the wheels its a faff to line the spacer with the wheel and then both with the hub to get the bolts in.
The more expensive ones bolt on to the hub first, then provide new fixing holes on the outer side of the spacer your existing wheel bolts screw in to.
I personally wouldn't touch the cheap type