Its primarily aero, first car does, all the rest slower cars need to stay in draft and follow. Aero reason, there is more drag when the car at high speed has to to push air against a nearby stationary object, then against free air, so, move away from the wall, if the corner line doesn't say otherwise. I'm sure there is some compromise involved, but the fact they do it says what they feel.

Its easy to visualize the amount of air we are discussing when watching the fence banners on th indy street road race circuits when the cars pass nearby at speed..