An air fryer is a compact convection oven with an unusually powerful fan and very little space between the element and the food. Understanding it that way explains both what it does well and where it disappoints.
Browning requires a dry surface
Food cannot exceed the boiling point of water while its surface is still wet, because the evaporating moisture carries heat away as fast as it arrives.
The browning reactions that produce roasted flavour and colour only begin once that surface moisture has gone and the temperature can climb past it.
A strong fan strips away the humid layer sitting against the food and replaces it with dry air, so the surface dries far sooner than it would in a still oven.
Small volume is the actual advantage
A domestic oven has a large cavity of air to heat and a lot of steel and glass to bring up to temperature, which takes many minutes.
An air fryer has a fraction of that mass, so it reaches working temperature quickly and recovers immediately after the basket is opened.
The short distance from element to food also means more radiant heat lands on the surface, adding to the drying effect the fan is producing.
Crowding defeats the whole mechanism
The fan only helps where air can actually move. Food piled into a heap has an interior that never sees moving air and simply steams in its own moisture.
Pieces touching each other have contact points that stay wet and pale, which is why results look patchy rather than uniformly poor.
A single layer with visible gaps is not a suggestion but the operating condition the appliance was designed around.
It is not frying, and the difference is chemical
Deep frying surrounds food with hot oil, which transfers heat far faster than air and penetrates crevices evenly.
Oil also participates in the flavour, breaking down slightly and carrying fat-soluble compounds into the crust in a way circulating air cannot reproduce.
The practical consequence is that wet batters, which set instantly in oil, drip and slump in an air fryer and never form a shell.
Where it genuinely outperforms an oven
Anything that was crisp and has gone soft, including previously fried food and baked pastry, revives well because reheating is largely a drying problem.
Small quantities of vegetables, coated proteins and dry-surfaced starches brown quickly without preheating a large cavity for a single portion.
Anything needing gentle even heat, a moist environment or room to rise belongs in a conventional oven, where the air is calmer and the space larger.