(Why) Do airliners travel together sometimes?
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.