When headphones have a built-in microphone, you can make a call to someone while you have your hands free to do other things. This is useful if you receive a call while cooking or when you're on a bike. Keep in mind that not every built-in microphone works equally well. Many microphones in headphones record a lot of noise. As a result, your conversation partner will hear you less well. If you really want to make hands-free calls, look for a Bluetooth headset.