If it doesn t distinguish between lean body weight and fat, why is body mass index so widely used?

Standardization. Body mass index is not a perfect tool to determine obesity, but it does allow for some normalization based on height and is easy to determine. Other methods such as underwater weight or electrical impedance require expensive equipment which is not readily available. Body measurements (waist, hip, neck) have been used but may vary based on the placement of the measuring tape.