From a comment as made by an acknowledged aviation authority:
"1/3 to 1/2 the war plane designs in WWI were pushers. Made it easier to sit a gunner/observer in front to do the photos and shoot the defensive guns without worrying about that prop."
At least during those most early stages of the Great War this was undeniably the case, there existing a terrible and apparently insurmountable problem with mounting weaponry on warplanes of the period.
From: "The Genesis of Fighter Aircraft"
"The first airplanes were not seen as offensive weapons, but as 'scouts'. "
"It soon became apparent that aircraft needed offensive firepower, and the best way to use it in combat was firing into the forward arc of travel. The obvious problem was painfully simple to see: firing a machine gun through the arc of the propeller would result in a shattered prop causing you and your aircraft sustain a fatal crash. Since the use of a parachute was not employed or condoned it was suicide and not combat. Several methods were tried, none of them were completely satisfactory."
A partial and initial solution to the arming of warplanes being:
"The Pusher Plane Option"
"Place the aircraft's powerplant behind the fuselage so the propeller pushes the plane instead of pulling it through the air. Now you can mount your gun in the nose. The bad news is pusher planes are not as fast or efficient as tractor planes.
The "pusher option" having also a glaring disadvantage:
"There is also the problem of crashing. Momentum would cause the engine to travel forward and act like a meat grinder when it catches up with the pilot." Pilots of the Great War eschewing the use of parachutes, a crash landing posing a greater danger than aerial combat!!
"Genesis" in addition describing in detail the final solution to the gun problem.
Here with a collection of images with commentary pusher aviation of the Great War [WW1] at least during that early period combat military aircraft functioning primarily in the role of scout/observer/reconnaissance/photographic:
A French Brueget 5. Two-seater with pilot in the rear, observer up front. Note the machine gun mounted forward of the observer. NO propeller in the way! Also the tricycle landing gear.
A French Farman Shorthorn MF-11.
A possible gun configuration for the Shorthorn. Observer in the front and pilot in the rear. I assume you would not have both guns mounted at the same time? Or if the pilot firing his machine gun when traversing from side to side you have to be careful as not to kill your own observer, saw the man in two so to speak with one accidental and unintended burst!
British DH-2a. This plane available in kit form full size and you too can own one and fly as in the days of the Great War, minus machine gun of course! This plane was a tail dragger the rear of the plane supported by a ladder for this posed photograph!
A British Vickers "gun bus" what it was called.
This merely a sampling of "pushers" as flown by the various combatants during WW1, the "pusher" a solution to a problem for which dynamics made such designs inferior, as described the forward firing machine gun firing THROUGH the propeller becoming all the vogue and the standard, much more lethal and versatile.