Have you learned how to lean the fuel/air mixture in your training airplane? Proper leaning at cruise altitude produces optimal engine performance. Leaning before you taxi prevents the spark plugs from fouling with lead deposits. Most important, a properly leaned engine yields the performance, range, and endurance figures found in the pilot’s operating handbook (POH). If you plan a flight based on those figures and don’t lean the mixture, you could run out of fuel.
When you adjust the fuel/air mixture, you are decreasing fuel flow to the engine to compensate for decreased air density at higher altitudes. Less fuel is needed for proper engine operation as altitude is gained.
Always follow the recommendations in your airplane’s handbook. These steps will yield a mixture that is rich of peak exhaust gas temperature in a normally aspirated, carbureted engine.