That's true that the initial charge is 4.2 volts. When you charge the whole pack, you want to make sure that a cell does not go above 3.6 volts. Now, how are you going to do that without a BMS ? Monitor each cell by hand with a volt meter ? Then when you find a cell that has reached 3.6 volts, turn off the charger ? This could be at 2:30 in the morning. That is what a BMS does for you. It monitors each cell and cuts off the charger when the first cell gets to 3.6 volts. It also works the other way. During discharging (driving) when a cell gets down to its low limit ~2.6 volts, the BMS sends a signal to cut back the controller to some slower speed so one can safely get off the road. Without BMS, a cell or cells can go down below their low limit and be destroyed before you know it. There are many kinds of BMS, from simple high and low monitoring to ones that have warning lights and buzzers, etc. to ones that also try to keep cells in balance. Prices range all over the map.
