I think it's borrowed from hockey, which makes it easier to compare the performance of different lines on the team.
"The plus/minus statistic was first used in the 1950s by the Montreal Canadiens. This NHL team used this ranking system for evaluating its own players. By the 1960s, other teams were also using this system. During the 1967-68 season, the NHL officially started using the plus/minus statistic."
https://www.liveabout.com/what-is-the-plus-minus-statistic-2779372
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