There are basically three reasons:

1. Safety
Cellular headsets improve safety by allowing both hands to be kept on the steering wheel. They also reduce radiation fears by keeping the phone antenna a safe distance from your head.

2. Economical
Cellular headset provide a hands-free option without the need for expensive equipment and a costly installation. Cellular headsets also shorten calls by reducing the need for both parties to repeat themselves

3. Sounds Better 
The closer the microphone is positioned to your mouth, the better it sounds. Cellular headsets that use noise canceling microphones also reduce distracting background noise