If they are not sold at the slightly higher price, the emergency exit seats are a bit roomier and are for Premium Plus customers, who fly with United over a certain number of times.
Typically the stewardess will ask the customer seated there, “If I’m incapacitated in an emergency, are you willing to open this door..?”
I don’t agree with this pricing plan. Never have. Although there is more room in the exit aisle(s), the seats do not recline.
That should be enough to maintain the price point, not charge extras.