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The positions 3, 2, and L encourage the transmission to downshift prematurely to 3rd, 2nd, and 1st gear respectively. This would be used in a situation where you want to use engine braking, such as going downhill for an extended period of time. By downshifting the trans and letting off the gas, the wheels are actually working against the force of the engine, and the car slows down. The Owners Manual explains the shift points in more detail.
Engine braking saves wear on your brakes. Not enough to make a habit out of shifting like this, but as I mentioned it's very useful going downhill in a mountainous area. If you were to ride your brakes for several miles, they'd overheat and possibly fail.
Semi-truck drivers love to use engine braking as much as possible - it's easier to do with a manual transmission. But because the engine revs higher and therefore gets louder than normal, you'll often see signs like "Engine Braking Prohibited" where there are residential areas near busier roads or interstates.
You didn't ask this, but for completeness, the position "4" is the current alternative to an "Overdrive Off" switch. Rather than put in the button, they added another shift position. With the shifter set on 4 rather than D, you will prevent the trans from shifting past 4 and into overdrive.