The gimbal does a pretty good job at covering up small imperfections, so most people will probably never notice if their props are a bit off balance. With balanced props, your Phantom will fly more efficiently. That could mean a longer flight time, smoother movements, more stable hovering, etc. It's the same reason people balance wheels on cars -- all in the name of efficiency.
Out of balance props can eat a great deal of performance as msinger mentioned.
Once you have a prop that is out of wack, you will be able to tell when launching your quad that 1st 20-30ft at lift-off. You can hear the difference, and in some cases see it at lift-off.
The most common way to what I call "warp the prop", is from packing in the hardcase. Even though there is soft foam it will compress and warp the prop for anyone that does not remove their props prior to loading the quad in a hardcase.
You can generally see a warped prop just by bringing the ends of the props together when mounted on the quad. If one end of the prop is higher then the opposite end, then something is not right.
You can also measure from each end to say the table top. But you need to set the landing gear on something so that the legs are not sitting on the foot pad cushions. They can easily be at different heights so placing something solid and away from the cushions will give the best results with that method.
Btw, it has been found that the P3 props do have the potential to fly off if only hand tightened. I like to hand tighten them and then use the tool to slightly add that just enough past the hand tighten stopping point.
There is 3 ways props are known to loosen up and fly off. One is ESC failure where the motor stops instantly and spins backwards. Another is by the brake which is when you are say fly forward at full speed and let go of the lever to slow down. The last is when say flying forward at full speed and then use reverse to slow the quad down. You can fly using the mentioned possible ways and never ever have a problem. But then again there is that chance something could happen.