Calculating weight and balance before each flight ensures that the weight is within limits and the load is distributed within limits as well.
Knowing what the aircraft weighs—and exactly where that weight is distributed—is important because both weight and balance significantly affect aircraft performance and handling. According to the Pilot’s Handbook of Aeronautical Knowledge, the center of gravity (CG) is “the point at which all the weight of the aircraft is concentrated. If the aircraft were supported at its exact CG, it would balance in any attitude.”
So, when determining how to load an aircraft, it’s important to consider the pros and cons of a forward or an aft CG.
In contrast, an aircraft with an aft CG requires nose-down trim. Trimming nose-down means less load on the wings, less drag, a faster cruise speed, and a lower stall speed, making the aircraft less controllable. This result is especially true at low airspeeds, and it is an important consideration because when the aircraft is less controllable, it could potentially lead to a stall/spin scenario that is difficult, even impossible, to recover from.
While forward and aft CG are along the longitudinal axis of an aircraft, stability-related issues can also affect the lateral axis (such as a fuel imbalance in the wings). Each aircraft’s handling characteristics are different, so check your airplane’s pilot's operating handbook for airplane-specific operational and performance information. Your flight instructor can help you understand the impact on performance for the aircraft you’re flying.
Use all available resources to make the best preflight decision regarding weight and balance before each flight. Weight and balance apps make it fairly simple to input various loading configurations to see what the impact on CG and performance would be under those conditions. You can also do this the “old-fashioned” way using the chart in your pilot’s operating handbook.