What is a bike fit?
The aim of a bike fit is to optimize a rider’s position on a bike to ensure maximum comfort and best delivery of power and therefore improve overall efficiency. Other factors are also studied in detail such as aerodynamics, but the overarching aim is to get the most out of the combination between rider and bike whilst reducing the risk of injury.
What's the point?
The whole point of a bike fit is to make you sit more comfortably (which will let you ride for longer) and to help you ride more efficiently. The transmission between the motor and the bike if you will.