In set 6847 (Space Dozer, http://www.peeron.com/inv/sets/6847-1) there is a black Technic Axle 6 that has no structural or cosmetic function in the set. I don't have the original instructions or box, so it might have been included to build one of the alternate sets. Or some other reason?
It's structural, but not really mandatory:
The full (4 pages) instruction can be found by googling or e.g. there: http://lego.brickinstructions.com/lego_instructions/set/6847/Space_Dozer
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.