It allows your cell phone to broadcast a weaker signal for a given (signal/noise ratio) situation. Many cell phones have at least two signal levels for broadcasting - e.g. 0.6 watts and 3 watts. If you're close to the tower, the lower transmit strength is adequate. That means it is less likely to affect to your body. It makes a difference to your cell phone's battery life as well.
Those "signal bars" on the cell phone displays aren't just of peripheral interest. They influence your cell phone's behavior.