I think that if you look at the picture that taul pall posted, you will see what looks like a simple light weight structure. A rounded structure CAN take a blow from any direction and disperse the load into the greater structure. It does offer better aerodynamics than a rectangular wing, it does hold up to the shrinking fabric covering better with less weight. With the rather pathetic engines of the era, weight was critical

This style of construction pictured by Paul has anti-compression struts, and positive and negative drag cables that are diagonal between the spars. Biplanes also allow bracing against the second wing which add strength again without much weight gain, albeit with loads of drag. A number of WW1 designs were not even true biplanes, with the lower wing used only to brace the upper wing. I believe the term was seqi-plane or something like that, where the lower wing has less than 50% of the area of the upper wing.