The idea of leaning the mixture is to allow the engine to produce maximum power. In the UK its not a big deal, but if you ever fly from any high DA airports then it can be vital to lean prior to take-off. [as in the Mixture - Best Power pre take-off checklist item].

Rgds
EA:D

Leaning the mixture is not necessarily, or even just, about producing best power, although it can be eg leaning for take-off at high density altitudes.

Best range or endurance is at least as important.

Financially, best economy by reducing fuel consumption is also a consideration.


Lean at all altitudes as long as the manufacturer's leaning parameters have been met.