If you have a 3-cell Lipo pack, and one cell is lower than the other two, and you charge all three in series (Like NiCad cells are charged) you run the risk of over-charging two of the cells - which could lead to a fire or explosion.

Most pack nowadays have two plugs. One is for running the airplane and it's in series. The other is a single ground, but it has a positive lead for each cell. When you use a balancer (or a balanced charger) the charger detects the amount of charge in each cell and charges them as they need it so that no one cell gets too much.