2019 2.5i Limited Forester (hers) (4th Subie), 2014 Impreza Premium (mine)(#5)
Not a guess, actual reality.Aerodynamics
Thanks for playing 'Ask the Subaru guy'.
In all seriousness, you know those little notch looking things on top of the wheel arch moldings? They're present not for aesthetics, but are just like these holes in the wheel wells, they're present for better aerodynamics. Minor details, and probably really not needed on the regular Outback or Legacy. On the Outback XT or Legacy XT - which now have sustained top speeds of well in excess of 100 mph, courtesy of the go-fast turbo ... needed.
One key thing to keep in mind - our cars specifically are ALWAYS designed with aviation type aerodynamics in mind. We can't help it, it's a major part of our heritage that we still embrace. (That's why we have airplane wings on the grilles of almost everything we make, just for your own trivia education.) And allowing better air flow through the wheel wells and yes, some brake cooling, allows for at least a slight improvement in the aerodynamics of the whole car. It's not much, it doesn't have to be much.