Why does the MacBook Air have five pins on the charger?

Solution 1:

From MagSafe Wikipedia page

Pin 1 - Ground
Pin 2 - V+ @ 16.5 V DC
Pin 3 - Charge control pin
Pin 4 - V+ @ 16.5 V DC
Pin 5 - Ground

• The inner large pins are V+ (16.5 VDC). Measuring with no load will give 6.86 V DC; the full 16.5 V is provided to the proper load.
• The outer large pins are ground.
• The tiny center pin is a charge control pin that assists with changing the LED color, as well as reporting the connected adapter type and serial number to the connected Mac.

Solution 2:

In other laptop chargers the pin is of round shape, so that there is no orientation of direction for the connection i.e. always +pin of charger connected to +ve pin of laptop and -ve to -ve.

In Apple's version out of 5 pins, 1 pin (middle) is for the battery status which provides a signal to LED to change color red/green.

As the connecter is of rectangular type, suppose if there were only two pins (+ and -) then there would be wrong connection possibility depend on the direction of pin connection. So to ensure correct connection in both way 4 pins are needed.