These workouts are based on linear progression. Put simply, linear progression means that if you were to graph your strength, the line that you would end up drawing would be straight. This means that you need to continually either add weight or reps every time you do an exercise. You can make progress very quickly this way if you’re a beginner because your body doesn’t need much stimulus to adapt. As you get more advanced, you need more stimulus and more time to adapt.
This allows for progressive overload. Without this, you won’t make any adaptations (strength gains or size gains). Simply put, in order to get bigger and stronger, you must continually make your muscles work harder than they’re used to. Conversely, if the demands on the target muscle groups are not at least maintained or are actually decreased, your muscles will atrophy, losing size and strength.
Progressive overload is a very simple but crucial concept, laying the foundation upon which successful resistance training is built.