Room to move
A group runs most efficiently when everyone rides close together. But in doing so you still need room to move. For example: The wind comes from the left, so you’re to the right of the rider in front of you. If the road then sightly leads to the right. The group as a whole will tend move towards the right and if you’re not careful you’ll be forced into the shoulder. The same happens if there’s oncoming traffic, and the group moves to the right in response to that.
You can anticipate many of these situations. You can avoid potential problems by ensuring that you do not get stuck in a rut, so to speak. You move back slightly so that when the rear wheel of your predecessor moves to the right, it moves in front of your front wheel, and it does not clip your front wheel.
At the end of a ride, a group may loose its close formation, and riders deviate more from their line than at the start of the ride. If you don’t let your front wheel overlap with the rear wheel in front of you, there’s no cause for concern. No matter how much that rear wheel moves laterally, it will not clip your front wheel. That’s not only reassuring for you, but also for the ones riding behind you.
When riding in an echelon, especially with crosswinds, you tend to ride with overlapping wheels. You can have reasons to not riding so closely together, creating some room to move (and precious reaction time), but still keeping the advantage of being on the lee side, though a little farther away.