The initial use was to ration advanced education (secondary or higher), so as not to lavish it on "morons" (at the time a technical term).
Similar in concept to the German Abitur, which originally tracked students into Gymnasium or vocational tracks. Today it comes at the end of a secondary education (high-school) programme, and is comparable to British A-levels.
<https://www.verywellmind.com/history-of-intelligence-testing...>