A diagonal subfloor will help stiffen the floor just as plywood has differentiated layers. If both the subfloor and the finish floor were oriented identically then there would be more chance of buckling and less weight bearing ability. The different direction of the grain adds stability.
In a wall the diagonal will solve racking problems if each half of the wall is oriented 90 degrees from the other half. If all of the boards are oriented the same across the complete wall it will greatly aggravate the problem.