Why two passenger airplanes travel side by side? Recently (2018-08-19 17:40 UTC) two planes flew above my city (Poznań, Poland). They were heading in the same direction, kept really close distance (side by side) - both flights to Riga:


https://www.flightradar24.com/data/flights/bt634
https://www.flightradar24.com/data/flights/bt684


Is this common? Why would they do that - are there any benefits of it?
The top one is Baltic 634, the bottom one BT684.

As you can see they had 2000ft seperation. That's 600m for us europeans not accustomed to aviation.

Furthermore, at 17:40:19 BT634 reported in at 52.409599,16.981899. 17:40:25 BT684 reported in at 52.423222,17.019314. While those are 6 seconds apart, the planes were north-eastern course at the time, so I guesstimate that they had 1-2km horizontal seperation as well.

As to why - they're going the same place. The separation is big enough that there's almost certainly no benefit to it.