it is just a function built into the game

when the villagers eat certain stews, they exercise

when they sucessfully complete a round of exercising, the game gives them building experience points for it

perhaps for keeping physically fit so they will be strong enough to be good builders!
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Why is it that if someone tells you that there are 1 billion stars in the universe you will believe them,
but if they tell you a wall has wet paint you will have to touch it to be sure?