VO2max is interesting as a measure of cardiorespiratory fitness (stroke volume, gas exchange, blood O2 carrying capacity, etc), but I don't know how they can factor out muscular efficiency. They don't know your blood volume or lung volume, to begin with. If I were to guess they look at energy over some time window at LT and derive metabolic threshold energy, and from that infer how much O2 is consumed at LT. I doubt that's better than a rough fitness estimate. For training it's not very useful (other than you want to keep your cardiorespiratory fitness pegged, or training will suffer - but that's easy, and beyond that... meh).