To add onto other’ answers about how acts break up the story into structures segments, there is also a logistical reason behind it. The stage crew and actors needed a break.
Because before the days of electric motors and hydraulic systems, absolutely everything had to be moved around the stage manually. The lights had to be turned on and off and raised and lowered manually (hell if you go back far enough the lights had to be LIT manually)
So doing a scene change was a bit more labor intensive.
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