A standard-capacity 30-round magazine keeps your AR-15 fed, but there's basically two conditions of which we can be aware: Full and Empty. Both are easy, binary conditions: Either the magazine has so much ammo you can't get any more, or there's none to be found.
When in use, though, there are myriad reasons why you might want a handy gauge between those two extremes. Whether you're battling a friend picking off orange clays at the 100 yard berm, taking a carbine class or in the middle of the zombie apocalypse (does anyone even talk about the zombie apocalypse any more?), being able to tell whether you're about to run out of ammo is a good thing.
Enter the Command Arms Accessories CD (Countdown) AR-15 magazine. It has a handy gauge on the bottom of the magazine facing you. There are two indicators: A numerical indicator based on the spring and follower position that gives a precise count of rounds remaining, and a three-part, color-coded background. If the magazine is full or has more than 20 rounds, the indicator is green; for 11-20 rounds it is yellow, and when the round count drops below 11 rounds it's in the red. A quick glance at the back of the mag tells you if it's time to perform a tactical reload or if you can wait for the next wave of zombies.