Why might an airliner fly a large U-turn before landing?
Airplanes prefer to land in a headwind. This reduces their groundspeed compared to landing with the same tailwind.

Braking distance grows quadratically compared to groundspeed. So any significant reduction in groundspeed is worth having.

Successive airplanes should all come from the same direction to land. This removes any danger of collision when a plane fails a landing and goes around into what would otherwise be oncoming air traffic. Planes also prefer to take off into  headwind, so if takeoffs are intermingled with landings (very common in a busy airport) they won't interfere with each other if they all go the same way.

The tower monitors the wind and then decides which way the airplanes should come in to land and directs landing craft to land from that direction.