I'd think it more an insurance on the Federation League team being good enough to go up, and the second-bottom Central League team really deserving to go down. Because relegating two in a 10-team (10-team?) league could be seen as a little harsh...
Secondary thought - if a team has a for:against ratio of 5:1 or greater (or even 4...) - maybe that's AUTOMATIC promotion??
As in, clearly too good for the league. Then you give the second-place team in the division the bullies came from the right to play-off against the bottom team from the one above.
Just revolutionising world football here...
Maybe also vice-versa for really bad teams. But not as important. I just hate it when teams/players are working below their standard.