What is the purpose of the Technic Axle 6 in set 6847?
When weight is applied on the structure, the wheels are are creating a twisting motion that will eventually dissasemble the 1x6 beams.



With the extra structural part when a load is applied there is a lot of flexing but the 1x6 beams are staying in place. 



Effectively, the axle is preventing the beams from rotating out of place.