The aircraft can have it's load distributed in any number of ways (for example which rows the people sit in, the number of them, where the cargo is loaded, how heavy it is, how much fuel is loaded and how it is distributed).
All of these factors affect the load and balance of the aircraft.
Once airborne, the speed you climb out at is a function of flap setting, power and weight. The last thing you want is to have the aircraft incorrectly trimmed - ie you don't want to have to input a control force just to get it into a trimmed condition, so all the loading factors result a the determination of the take of trim setting which should result in the aircraft stable in pitch after take off.
Modern aircraft have such a wide range of centre of gravities available to them, that the wrong take off trim setting could result in there being insufficient elevator authority available to get the aircraft airborne.
Does this help?
Rgds TopBunk