Whatchou talkin bout Willis?
You don't need a machine to record BMI. Unless you count a scale and a ruler as "expensive machines".
BMI is just a function of weight vs height. You mean bodyfat percentage, right?
No, advanced machines give body composition reports. They shouldn't call them "BMI machines," but they do. Maybe the doctors aren't calling them the right thing. They are body composition machines, but the majority of the time the doctors just call it "getting a BMI." Maybe it's easy to say.
It tells percentage of and pounds of fat, ECM (fluids) and BCM (muscle). This is useful because you know what you're gaining and/or losing, of course. It's fun, because you can experiment with your training and figure out what ACTUALLY yields the most muscle. For instance, you can run 2-month experiments on yourself, or even 1 month.