[kokken]

The reason why people form two lines to board the Marunouchi line in Ikebukuro is not because they prioritize seating, or going as soon as possible.

The true reason is that the line forks into two lines later on, with the final station being either Honancho or Ogikubo. The second line that form is just for the people whose destination would not be served by the incoming train.

[MarkusQ]

Would be interested in a source for this claim, but it from the map it at least looks reasonable.

https://en.wikipedia.org/wiki/Marunouchi_Line#/media/File:To...