X-men - the movie which shows us extraordinary powers! If you look close, you will see that most of the characters go through some emotional trauma and then their physical form goes through changes that make them achieve those special powers.
Tears are something like that. They will not help you create storm or grow claws in your fists but are very useful. Apart from the biological function of clearing eyes, tears are like safety valves for our physical form.
Imagine a pressure cooker with no outlet and constant heating. It is bound to explode. In the same way, we Humans have high emotional quotient and if we do not have a way release to the same, there are so many things that can go wrong with us - Loose balance of mind, might hamper nerves in our brain and there by affect us physically.
Tears help ease out the pain in our heart and mind when let out of the system. Usually happens when we do not have control on the events in front of us and the anger / frustration / sadness etc.
So, next time you feel like crying or tears roll down your eyes, know that it is not a sign of weakness but a sign of restraint.
If you see someone else in tears, try and lend a shoulder. You never know when you might need one!