The Importance of Progression
Imagine being in the middle of a lake, why you are there and how you got there are not important (but I assume that it would be a great story). You have two options. One is to tread water, keep your head above the water and stay alive. The second option is to swim to the shore (Actually, there are three options. The previous two plus drowning). Without progression, the third option is enviable.
When it comes to fitness, there are a couple of principles that must be followed to achieve results. Among them are intensity, overload, and improving. When properly implemented, these three principles will allow people to reach their goals in a timelier manner.
Intensity means you need to work hard. Overload means that you have to place a little more stress on the muscles than they are used to. Progression is gradually adding weight, repetition or time under load to maintain the overload. You can make the argument that it is the most important principle. For example, without it, after a few workouts, there is no overload. Without progression, after a few workouts intensity will decrease. Without progression, you might as well stay home!
Back to the middle of the lake. Finding yourself in the middle of a lake can certainly be an intense situation and treading water to stay afloat will surely overload your muscles, but the only way to show progression is to swim to shore!