What were airplanes first used for in WW I?
Artillery observation and reconnaissance.
Balloons had been previously used for Artillery spotting and Battlefield reconnaissance. The airplane became the next evolution of observation. Just as soldiers began to shoot at Balloons when they figured out what they were for, soldiers began to shoot at airplanes. Anti air also evolved.
The first sorties took place almost immediately on the start of the War reporting on German troop movements.