Probably more for safety but I notice often the great arch flight paths from US to EU often tend to have bunches of planes grouped up. Nowhere near close enough for any fuel gains, I expect, but it was an interesting quirk I noticed recently.

You see that stuff a lot on flight radar. There's a mini formation going over DC right now, headed SW, Charolette, New Orleans, Orlando, Charolette again, each 10 miles apart from each other. That makes sense on the Easter seaboard, scheduling large packet flows, jumbo frames of jumbo jets. It was a little more surprising to notice clustering happening across the Atlantic.