OceanMist333, I'll give you the (dubious) benefit of the doubt, and shudder to think that anyone let you get through divemaster training without making this extremely clear.
A lot of people are gas-limited for their dives -- in other words, on the tanks they are using, they simply CAN'T get very far into decompression before having to surface because they have gotten too low on gas to remain. A good-sized man diving an Al80 is an example of this. At 100 feet, and assuming a reasonable SAC rate of .75 cu ft/min, you'll use 3 cu ft of gas per minute -- so at 20 minutes, you'll be down to about 750 psi and more than ready to head up.
But if you dive big tanks, as a lot of us do in our area, it's easy to carry enough gas to get well into the mandatory decompression range. I did a dive two days ago on 32% Nitrox, where we went into mandatory decompression, if only by a little bit. Knowing exactly how much decompression we owed, so that we could design our ascent profile correctly, was important. There is a difference between being one minute over your NDLs, and being 15 minutes over, and you have to handle the ascents differently or you WILL feel like you've been run over by a truck . . . if you're lucky.
If you are feeling completely beat after your dives, I would highly recommend taking a second look at your gas choice and your ascent protocols. What the DIR world calls "minimum deco" tends to make most people feel a whole lot better and more energetic after their dives . . . and ready to do a second, or third, and maximize the enjoyment of our underwater world.