Found this photo of a 6800 crank cutaway. This puts more material on the leading and trailing edges and less on the sides, but being a tube it resists torsion better than an i-beam or solid beam. Same reason as all the other tubes your bike is made of.
The outer chain ring is also hollow monocoque construction This is a cutaway view of a DA crankset, showing all three hollow things (spindle, crank arm, sprocket)
The hollow spindle is a good system. The arms, well... Based on what I've learned finding those cutaways, I've suddenly lost interest in a hollow-arm crank like this. There are many more than zero reports like this one.
failure shimano ultegra crank causes serious bicycle accident
