BMI is a measure of body fat based on your weight in relation to your height. The reason doctors and trainers use BMI is it is the best measure of body fat for the human body. It is however, not always accurate.
Why use Body Mass Index if it is not accurate? What’s the point. Everyone needs tools. BMI is one tool that is used to determine how “fit” you are. It should not be the only thing that is used. If your trainer or doctor is obsessing about the number and not looking at all the factors you may need to get a second opinion.
Women tend to have more body fat than men. There are several other problems with the BMI measurement. It does not take into account the differences between ages, or muscle mass. Those who are muscular may have a higher BMI than their thinner less fit friends.
The BMI also does not account for those lactating, pregnant, or have not reached full maturity.
In short, your BMI is one of many things to take into account on your fitness journey.