Why must the dump truck operator climb into the back of the dump truck?

There are several reasons why they would:

• Leveling out an uneven load
• Cleaning out the bed (particularly if the truck has been hauling asphalt or concrete)
• Spraying the bed down before loading with asphalt. Dump truck drivers hauling asphalt will often spray the bed down with either something like Black Magic, or else diesel fuel, to ensure that the asphalt does not stick to the bed.
• Pulling on a tarp which won't unravel properly.
• Removing a small amount of material (typically with a shovel) which doesn't justify opening the tailgate and raising the dump bed.
• Inspecting the dump body or bed liner (if so equipped) for damage

Things like that.