While browsing through the planes in FlightRadar (once again), I've noticed that sometimes some airliners from the same airline company seem to travel "together" on long distance flights to destinations located relatively close to each other (Like Finnair from Helsinki to Hong Kong and to Singapore or to different Japanese cities). The planes were separated vertically by about 2000ft but otherwise quite close to each other. Is there any advantage to this or a good reason? Or is this a pure coincidence?
They do this to allow convenient transfer times for passengers at major hubs (in this specific case, Helsinki). In the hours up to the departures of these long range flights, many short- and medium range feeder flights will arrive at Helsinki, with a lot of passengers who are transferring to the long range flights. It basically enables them to operate an efficient spoke-hub model.
Looking at flight schedules for Helsinki as an example, a lot of transfer passengers from all across Europe will arrive on these flights between 15:00 and 16:00:
Major European arrivals
15:00 Budapest and Berlin
15:10 Dusseldorf, Frankfurt and Manchester
15:15 Brussels, London and Amsterdam
15:40 Oslo and Nice
15:45 Warsaw and Rome
And will then transfer to the various long distance flights that depart between 17:00 and 17:35:
Long distance departures
17:00 AY67 to Guangzhou
17:15 AY73 to Tokyo and AY79 to Nagoya
17:20 AY57 to Shanghai
17:25 AY77 to Osaka
17:30 AY41 to Seoul
17:35 AY89 to Bangkok
Because the long distance departures are bundles so closely, it is possible to create short transfer times from all major European airports to any of the long distance flights.
I do not have inside knowledge of Finnair operations, but I would expect that they have a limited number of staff who speak Japanese, and so logistically running the check in desks for several flights to Japan at the same time allows the airline to schedule checkin staff with appropriate language skills to cover the appropriate flights.
There are also restrictions at both Helsinki and in many Japanese airports that restrict night operations and aircraft on the ground overnight do not earn the airline money. As a result, there is a limited range of flight times that ensure that the aircraft are operating continually.