Costs and weight I'd say. Somebody who understands material science better than me will likely correct me, but a rectangular cross section (with the long edge vertical) will be stiffer than a square cross section of similar cross sectional area. So I'd rather use a 2x4 in that fashion than a 3x3 for the same/similar volume/weight of timber.
Costs and weight I'd say. Somebody who understands material science better than me will likely correct me, but a rectangular cross section (with the long edge vertical) will be stiffer than a square cross section of similar cross sectional area. So I'd rather use a 2x4 in that fashion than a 3x3 for the same/similar volume/weight of timber.