If you buy a 30MPG car your not going to actually get 30MPG as it will depend on your driving habits. However, it's based on a meaningful test so you can do meaningful comparisons.
As it stands an ISP can advertise 100 mbps service and fail to show Netflix streams when a 10 mbps connection on another ISP can easily handle 2 of them in HD. Thus, consumers need something meaningful.
PS: Picture trying to compare gas MPG if car companies could report MPG while costing down the side of a mountain. That's a perfect recipe for Honda to optimize the wrong things just like ISP's do now.