It might be confusing to understand but muscles have memory. Joe Weider was well aware of this phenomena and as a result introduced the principle of muscle confusion.
Muscles learn the sequence of a physical endeavor and fire certain motors nerves accordingly to work as efficiently as possible. As you begin to learn a new physical skill you progressively become more and more efficient at that movement. The same effect occurs during your workout.
As you begin to repeat the same workout your muscles start remembering the variables of the workout, such as the order of exercises, the amount of rest between sets, the rep ranges used, rep speed, volume, intensity, and etc.
While your muscle are learning the workout they are under a considerable amount of stress. Once complete motor learning has occurred the results you obtain from this workout slowly start to diminish.
In order to keep things moving it is important to mix things up. Instead of giving your muscles a chance to fully adapt to your workouts throw in something different so the muscles never quite know what to expect.
For example, instead of doing bench press followed by incline press followed by flyes. The order can be reversed. The flyes will pre-exhaust your chest and the strength of the arms and shoulder can be used to further punish the chest – leaving the pectorals muscles screaming.
Instead of using heavy weight and resting long between sets – drop the weight and try only resting 30 seconds between sets and no more than a minutes between exercises.
Which variables you decide to change does not matter. Just as long as you ensure something is being continuously changed in your workout. The bottom line is that your muscles will not adapt to your workouts and gains will not start to diminish.