It's not for loading, it's to stop movement that then causes sideways strain on bolts.
As like you said bolts are to pull the two faces together, not stop sideways movement.
These answers always make me wonder what people think to loads due to braking and acceleration which are axial to any spigot?
I have read so many times that it is to prevent loading on the studs / bolts. The fixings, when correctly torqued, are in tension and create static frictional loads between the wheel and the hub face (Making it particularly important to ensure repainted wheels do not have paint on the hub face. Which is responsible for more broken fixings and warped discs that anything else)
Depending on the type of fixing used, which in most alloys is a conical seating, although some, like Jaguar use a shoulder nut design. The centring action of the cone of the fixing mating with the conical seat in the wheel pulls the mounting hole in the wheel concentric to the bolt. It is the correct tension in the fixings that is the priority.
It is best shown in pictures. While this is the HGV inspection manual, the principal is identical. The cone on the fixing is what pulls the wheel into position. As seen there is no spigot for either the conical or spherical. In the double wheel conical fixing, the drive load and support is carried by the larger diameter conical part of the stud. The outer wheel is then clamped to the inner wheel and centred by the outer nut and the loads are transferred from the outer wheel to the inner wheel via the clamping force. It is only when using flat face or washer faced fixing, that have no centring ability, that it necessary to have a spigot.
Regardless of any centring method if the fixings or spigot is subject to radial or shear loads then the fixings are not tight, damaged or incorrect and then you really will have a problem.
The two cone principal is also exactly how spline drive "knock off" centre lock wheels work. With wire wheels, this was pretty much the only way to drive the wheels. But while this was a large centre fixing, the principal is exactly the same for conical fixings. In fact it is not much of a stretch to see this as a lager version of a mounting bold.