Hi Floyd,
The short answer is safety. If one cell should age more than the others and loose capacity as compared to its pack-mates, then it is possible to overcharge some cells and undercharge others when charging the pack to a finish voltage of "N" times 4.2 Volts. What if thre of four cells in the pack ends up at 4.1V and one ends up at 4.5? It's called venting with flame.

The balancing is not needed after the cell has been discharged, but rather during and especially toward the end of charging. For this reason, purpose-built balancing chargers are preferrable, but not mandatory. Chargers with separate balancers work fine, provided they are of the type that has a connection to the charger, allowing it to either throttle back or disable the charger if the balancer is overwhelmed. Typically these work by shunting some fixed or maximum charging current "around" the cell and if that current is less than the charging current then bad imbalances will cause this condition. I like the FMA chargers, myself.

take care,
Dean