I've seen some trucks (such as dump trucks) with an axle that can be lifted up and down. My question is, what is the point of that? If they left it down all the time, why couldn't they just keep it down? What would be the reason to engineer that into a truck? If they kept it a solid axle, wouldn't that decrease the complexity of the truck itself?

Also, would it count as an extra axle when it's down (say when paying toll for a bridge)?