Even if it’s not an MMO, player count can be a very strong indicator of enjoyment/income at a given time. Even if it’s a single player game, player count can show how popular a game is on whole. If a 5yo game still has engagement numbers above newly released games, it’s strongly correlated with studio income/gamer trends.
They can base future decisions on what they did correctly/incorrectly, and develop their next game/dlc/etc with those lessons learned.
Release a game that has nobody playing after a month? Crapbasket. Release a game that has well into a million players despite the age? Fucking masterpiece.