Isn't it just so they can put 4 wheels per axle? I could be wrong here.

If I understand your question correctly, then I should say that it is partly for reasons of strength and weight (a domed structure can as a rule be made stronger at a given thickness than a flat structure of the same weight) and partly to achieve either a wider or narrower axle track (the distance between the wheels on the same axle) than is practical with a given length of axle.

By turning the concave side of the wheel towards the outside, one can widen the axle track, and vice versa.

On any given design of vehicle, or sometimes even on different axles of the same vehicle, the requirement for mounting the wheels one way or the other may differ.

I may be wrong but, I have always thought that the shape of wheels on vehicles was such that the center of the tire tread where it touches the road is kept directly below the point about which the front wheel turns for stearing purposes. This would ensure that the wheels does not pull outwards or inwards either under acceleration or braking. For the purposes of carrying a spare it makes sense to have all wheels the same.

On a vehicle that requires two wheels each side of an axle to carry heavy loads, the inner one can be placed "right way round" and the outer one the other way round to enable the same set of threaded studs to be used to fix both wheels.

Stewart's point about getting the axis of steering to meet the mid-point of the tread at road level is correct. This is absolutely critical when you hit the brakes, as otherwise the braking forces would try to turn the front wheels inwards, doing no good to the track rods.

The wheel also has to be arranged so that the bearings inside the hub share the load equally. So the outer bearing is normally in line with the outer side of the tread, the wheel dish and the end of the axle, and the inner bearing is along the axle in line with the inner side of the wheel. So the hub is about the same width as the tread.

Pragmatically, the dish of the wheels allows the brakes to be stuffed inside the cavity, so the suspension can be very close to the wheel to avoid long cantilevers, and so the inside width of the vehicle is not compromised.

But mainly, Pete's right. If you have a deeper dish on the wheels, you can mount them back-to-back on the rear axle(s) and have six identical wheels on a 2-front, 4-back layout.

Only  one type of wheel is used, but the flange is dished and the wheel will therefore appear to be concave or convex depending on which way it is fitted on the end of the axle (which is called the hub). The basic rule is the first wheel onto a hub is always concave inwards.  The second rule is that if you see a concave wheel from the outside, it means there are four wheels on that that axle.

For front wheels concave-inwards places the tread surface more medial relative to the end of the front axle and therefore minimises moments (hence stresses) during steering and braking. These wheels will appear convex from outside. For rear wheels the issue is increasing the load-bearing capacity by having more wheels on the same axle. Here the inboard wheel is again fitted concave inwards, and the outboard wheel is turned around to be concave outwards. This permits the same wheel studs to be used for fastening: efficiency. The flange is so designed that it protrudes slightly beyond the side-wall of the tyre, so that when the wheels are fitted back-to-back the tyres will not interfere and damage each other when they deflect under load. Front axles almost never have double wheels on each side, at least not on road trucks, because of the steering difficulty that would cause.

If greater load-bearing capacity is needed at front  then the solution is to have two steered axles, each with one wheel on each hub (four steered wheels total). Multiple rear axles (typically two) is also a common design for tractor units and their semi-trailers, and these generally have four wheels per axle. But some buses with twin rear axles have only two wheels on one of these and four on the other. So if you watch this go past you will see convex-concave-convex! This still obeys the two rules above.