BMI, or Body Mass Index, is used primarily in health assessments to categorize individuals based on their body weight relative to height. It helps determine whether a person is underweight, normal weight, overweight, or obese, providing an initial assessment of health risk related to weight. However, BMI does not account for muscle mass, body fat distribution, or metabolic factors, so it should be used alongside other assessments for a comprehensive understanding of an individual's health status.