Why heavy vehicles have double tires at rear axels?

Example 1: - Suppose you've to lift a chair. You can do that all by yourself. But what happens if we replace the chair with a Sofa. Well you’ll have to call a friend or two to help you lift that thing up. Okay, now, let’s replace the Sofa with a chair again, but this time the chair is as heavy as the Sofa. You’ll still need them friends to help you out in this lifting.

Example 2: - Have you wondered why knives are sharp and Bag straps are kept wide. If you have used knives, you must have realised that with a sharper knife you need to apply very minimal force to cut through an object. But with a blunt one, your hands get tired chopping an onion/potato. On the other hand, when you go backpack shopping, you must have always seen that it is easier to carry heavy loads in a backpack with wide straps as compared to the ones with slimmer straps. Indeed the slimmer straps start hurting your shoulders.

In the above two examples we realise two things: -

1. Load sharing: - The more number of tires/wheels under a heavy vehicle are friends of each other. They help each other by sharing the load amongst each other and providing stability to the large carriage area and the large load above them. This ensures longer life for each of those tires and more stability to the load being carried.
2. Pressure Minimisation: - We know that Pressure=ForceArea$Pressure={\displaystyle \frac{Force}{Area}}$ . So when you went for buying that knife you looked for the sharpest of them all because that would ensure that the area of application of force is minimum and thus you have to apply very minimal Force to get the same amount of pressure. Similarly, when you were buying the backpack, you looked for the wider straps because the wider straps with increased area, reduces the pressure on your shoulders, thus reducing the hurt. In the same manner, larger number of tyres provide a larger contact patch with the road, thus the pressure due to the weight of the load on the road is reduced because of larger number of tyres.

One might say, why can’t we instead use wider tyres. Well the double tyres have their own benefits, for example, say if there is a tyre burst, a whole wider (more expensive) tyre is blown up then, totally compromising one side of the vehicle. With the doubled up tyres, a) the tyres are cheaper, b) give more reliability as in if there is a tyre burst on one of the tyres, the whole vehicle still has a leg to limp on to the garage or to a halt on the side of the road for a repair.

So the larger number of tyres are there for 3 primary reasons - 
1. Load Sharing between tyres to improve longevity and provide mechanical stability to the loading bay. 2. To reduce the pressure on the road and not damage it and also reduce pressure on themselves to not damage themselves. And 
3. To provide stability in case of emergency situations such that the driver has time to react and bring the vehicle to a halt in a safe place.

Reason Number 1:
Dual tires are used on the non-steering axles of heavy-duty commercial trucks to increase their load capacity and help maintain vehicle drivability in the event of a flat rear tire. Placing two tires on both ends of a single axle nearly doubles the weight that the axle can carry and allows three properly inflated tires to temporarily carry the weight originally allocated to the four tires if a rear tire loses pressure or goes flat.

Reason Number 2:
The power of the engine goes to the rear wheels. Having extra traction in the back by having extra tyres back there will help it to maintain stability.

WHY NOT DUAL TIRES ON STEERING AXLE:

Putting dual tires on a steering axle will result in increased grip with the road that would greatly increase the effort required to turn the wheels, and would result in a high rate of tire wear.