It doesn't really matter one way or the other to me. But that has nothing to do with the Invisible Jet or Robot Plane. Superman should have his own spaceship. Abin Sur travelled in a spaceship despite having a power ring. The Thanagarian Hawks had their own spaceship and flying is their main deal.
People can walk under their own power--that doesn't stop them from using buses, trains, cars, bicycles and skateboards.
If Diana is going to have a supporting team--like she had in the first movie--then she'd be a bad leader if she flew away on her own and left the rest of them on the ground to walk. Having an airplane with stealth capabilties allows her team to do a lot more.