Just simple, Simulate reality:
1.- Walkaround mode, with oil on the ground below the plane, oil in the cowlings, bugs in the wings and windscreen, possibility to check lights, fuel quantity visually....
2.- Taxiways have slope, runways have slopes, have different textures /patches, water / snow accumulates, grass is uneven, there are bumps when taxiing, rolling, plane skids...
3.- ATMOSPHERIC MODEL! Not only plane´s: You have wind gusts, wind gradient, wind climbing an slope =updraft, tarmac, hangars, asphalt, cement, rocks warm with the sun and create thermals, we don´t fly on rails, plane wobbles, vibrates, suddenly your right wing rises, then you feel a bump, then you start to sink a bit... there is icing, wave, rotors....
4.- Planes Fail: Alternator fails, fuel pump, Engine leaks oil and you can see on your windscreen, landing lights go dark, a door can open suddenly, landing gear unoperable, engine fire, icing, lighting strike, bird strikes...