What are these shelves inside the bed for?

[dmjlambert]

Yes, interesting.
I downloaded a copy of the owners manual today and found a part about Two-Tiered Loading:

Two-Tiered Loading
By positioning four 2” x 6” wooden planks across the width of thepickup box, you can create an upper load platform. The planks must be inserted in the pickup box depressions. The length of the planks must allow for at least 3/4“ bearing surface on each end of the plank.
When using this upper load platform, be sure the load is securely tied down to prevent it from shifting. The load’s center of gravity should be positioned in a zone over the rear axle. The zone is located in the area between the
front of each fenderwell and the rear of each fenderwell. The center of gravity height must not extend above the top of the pickup box flareboard.