Question: why kg is used for weight?
Answer: When we use kilograms to measure weight, we are actually referring to kgf or kilogram-force. From Wikipedia: One kilogram-force is equal to the magnitude of the force exerted by one kilogram of mass in a 9.80665 m/s2 gravitational field. In other words, the weight(force) of one kg is equal to one kgf, or 9.8N.