The idea of a turbo mode is that you'd normally need less light than that, but once in a while, you might want a bit more juice to make out something you could not see with the lesser amount, etc.
Its like having glasses, and a set of binoculars. You can see further with the binoculars, but, normally, you see what you need to with just your glasses. Once in a while, you need to see further, you raise the binoculars up, see what you need to, and lower them again.
If you are always on turbo, it means you should have gotten a more powerful light to start with for example.
The reason its called turbo and not just "High" is because it damages the light to stay at that screaming setting for too long, and you, or the electronics, needs to throttle it back to improve durability/survival when its getting too hot....and like the turbo in a car engine...you typically would not be running on turbo boost your entire drive to work and back, maybe just to pull out off the exit ramp/pass that semi, etc. (Unless you have a Turbo'd Rotary, etc...)
High or below that would be what the light can sustain with 100% duty cycle...and Turbo would be what it could sustain for a limited duty cycle.