Have you ever been in an area where you are positive that there is signal, but you are getting nothing (insert another AT&T joke here)? Some choose to restart their iPhone, but that can take a few minutes depending on factors such as the version of iOS you are using, the Apps, the model iPhone you have, etc. Nobody wants to spend 5 minutes waiting for their iPhone to start up, right? Didn't think so.
The next best thing when you need to find that cellular signal is go to your Settings. Turn Airplane Mode on, to disable your cellular data. Once it's on, turn it off again. This will restart it and will many times find your cellular signal again. That's much faster than restarting your phone.