I see a lot of trucks, such as this concrete mixer, and it’s been bugging me; they have a lot of tires, some are up and others are down...why?

I figure it’s one of three things. The first idea is that they perhaps rotate which tires are used (if it was down its now up, and vice versa) because the strain and weight of the cargo kills tires quicker than the front does. But I have never seen a truck like that.

The second and more plausible idea, is that the heavier the load, they drop more tires. But again, I have never seen that either.

The third idea is that if any of the road-contacting tires explode, they can drop an unused tire to help support the load.

How close am I? I've thought about this a lot and these were the best answers I could come up with. Help quench my thirst of knowledge Oppo!