The purpose of solution heat treating is to provide enough thermal energy to dissolve, in a solid solution, the alloy elements present. Without getting into the solid state physics of the metallurgical reactions, dissolution does occur but only at this high temperature. However, if you slowly cool down the part, the copper wants to come back out of solution.
Here is where the important step of quenching takes place. Quenching is a very rapid cool-down, usually using water, at a rate of 260°C to 315°C (500°F to 600°F) per second. Quenching locks in place all alloy elements that have been dissolved at the high solution heat treat temperature. Before the alloy additions can think about changing places and moving back out of solution, they are locked in place by the rapid quench cool-down. The result is called a ‘supersaturated solid solution’, an unstable condition. Quenching is critical to proper solution heat treatment.