BMI provides a significantly more accurate representation of body fat content than simply measuring a persons weight.
BMI is a statistical tool that works well for populations but should never be used by itself for individuals are there are far too many variables. My insurer has a standard BMI test - exceed it and they require a medical to continue to get the "healthy" discount. Pass the medical and they pay for it. So it is used as a way of splitting the population but they understand its flaws so don't use it as an ultimate test. Any medic (or health provider) who does is utterly incompetent and should never have got their licence.
From an individual perspective a better test is whether your waist is greater than half your height ... if it is then you are probably carrying too much fat. It still isn't a great test but is better than BMI as it is more tuned to individuals rather than populations.