So what is an air fryer? It is essentially a counter-top fan oven, usually looking somewhat like a deep fat fryer. Your food does not actually fry, but the size shape and high heat effectively gives you the same effect, but with a lot less fat. You cook food from scratch in it, but it is also particularly good at things like fish in batter or breadcrumbs and oven chips.
Why not use a normal oven? Convenience and quickness, really. I have a twin oven cooker, which can probably cook most things I can do in the air-fryer. But the air-fryer takes only 3 minutes to reach cooking temperature, against my oven’s 10-20 minutes; the temperature is far more precise, and will tell you when the cooking time is up; it is easier to keep an eye on the food, and much easier to clean up afterwards.