Biplanes became the first airplanes to enter service due to their structurally efficient design being more suitable for the weak materials then. This efficiency comes from the two wings acting as the chords of a very deep truss.
Another structurally favorable factor was the reduced wingspan and thus moment allowed by distributing lift across stacked wings.
Yet another weight saver was the reduction of wing loading, which enabled thus use of less powerful engines for slower flight.
However, the extra wing only produces 20% more lift with 100% parasitic drag increase. Ironically, this interference drag is even more severe at the lower speeds which biplanes are optimised for.
A braced monoplane has most of the structural advantages of a biplane, with less of the aerodynamic disadvantages. So it would seem to be a more logical design than a biplane, given the same materials and engine.
Yet biplanes were built in large numbers. Why?