The IAT reads the Intake Air Temperature. It then passes this value to the computer, so the computer may adjust timing and ignition based on the temperature of the air going into the motor.
Now, you say, "What about the MAF? Doesn't it have a temperature sensor on it?" Well, yes and no. The little temperature sensor (actually, more liek a heated fillament), on the maf works in conjunction with another part of the MAF sensor to measure how much air is going into the engine.
In the grand scheme of things, it really doesn't matter where you put the temperature sensor along the intake tube. At 3000RPM that air is moving so fast it only takes fractions of a second to go from the MAF to the cylinders.
Temperature ain't going to increase dramatically in that short period of time.