It's almost certain the secret mission was to capture a satellite in a single orbit, that's the only reason for the wings to be so big, in addition to the cargo

It's almost certain the secret mission was to capture a satellite in a single orbit, that's the only reason for the wings to be so big, in addition to the cargo space. If you're in space for a long time, you can time your deorbit so that you're landing close to KSC, but in a single orbit Earth's rotation gets you about 20 degrees away, so you need good gliding and steering to get back. Scott Manley has a good video about this.