1) I don't think the scheduling function is available in all markets. If it's not showing up and you're using the latest app, you're probably not in a market where it's available.
2) When you schedule, you're not guaranteed a pickup time. They give you an approximate window, but you're still completely at the whim of car availability (and surge pricing) at that time. It doesn't schedule a car to be near you, it just sends out a request at that time the same as if you sent it out manually, and if there aren't cars in the area, you won't get a car. For rides to the airport like this, I greatly prefer to use a traditional scheduled car service where they actually arrange in advance for a specific driver to be at your location at the scheduled time.