Why did they used to bleed people?

Bloodletting, an ancient medical practice, was based on the belief that imbalances in the body's humors (blood, phlegm, black bile, and yellow bile) caused illness. Physicians used to bleed patients to restore this balance, thinking it would cure various ailments. This practice persisted for centuries despite its lack of scientific basis, as it was ingrained in medical traditions. It wasn't until the development of modern medicine that bloodletting was largely abandoned.