Tyre balancing or wheel balancing is required when you feel there is a vibration on steering wheel or comfortableness drive after certain speed limit (in general around 60km..one can easily notice.)
if the tyres or wheels are not balanced there will be negative implications like it may effect the performance of vehicle , uncomfort driving, vibrations in steering, premature wearing of tyres and also wearing of suspension parts etc
So It is always advised to check with the technicians one you notice vibrations in steering. During the wheel balancing process the technicians check the wheel by mounting on balancing machine which locates heavier part of the wheel that causes the wheel imbalance. Afterfinding the amount of weight , they attach equavalent lead weight to the other wheel to balance it.
Also we can easily notice the imbalances whether it is in front or back wheels depending on vibrations. If vibrations are in steering , problem is in front wheels or if vibrations are felt in seat probably in rear wheels.