“First division” and “second division” recall a time when there were only 8 teams per league and the top 4 teams in each league got a share of the World Series gate receipts. For most players, a Series winner’s or loser’s share could be a substantial part of their overall earnings, and even for 2/3/4 place, it would be a nice bonus. One team might run away with the pennant, but there was still some money at stake for the others. After divisional play started in 1969, the terms mostly faded into irrelevance.
https://en.wikipedia.org/wiki/First_division_(baseball)