Originally Posted by Learn_not2burn

It intuitively makes sense, but if you do the actual math with moment of inertias about the centroid of the beam, it proves to be the optimal case. It makes it stronger in the direction when it bends from your weight, and lighter by removing material from where you don't need it, on the sides (because you don't need to worry as much about flexing side to side). I actually brought in my seatpost to show some of the students in the Mechanics of Materials class I'm TA'ing right now.