In order for you to stimulate changes in your body through exercise, you have to force your body to attempt things it is not able to do in its current condition. For example, if you can perform 8 repetitions with 50 pounds in a certain exercise, and you never attempt a 9th repetition, why would your body ever alter itself?
It won’t, because it has no reason to. You need to workout at or near your existing capacity in order to stimulate changes, and the harder you workout, the less time you can or should spend working out. Just as no human being can sprint all-out for a mile, no human being is going to get stronger, leaner and shapelier by performing workouts that are too long and too easy.