When you are at a T junction (where you can turn right or left). You’re waiting to turn right, and someone approaches from your left, and races up so that you can’t go, and then turns across you to travel away from you along the road youve just come down.

What did they Gain? You’re going a different direction so you’ve not got in front of them, and by racing up they’ve wasted more fuel and brake pads, but so many are desperate to make the turn before you, even though flashing you out as they approach is courteous and sensible.