There are many reasons for using Li-Po batteries in Airsoft. The biggest reasons to use a Li-Po is its smaller size and better power efficiency. Unlike traditional batteries, Li-Po gives a stable Rate of Fire, through-out its use. Also, it is not necessary to discharge Li-Po batteries before charging them, allowing the battery to be charged in-between games.
However Li-Po batteries are more delicate and requires more care in charging and handling. It is recommended to charge the battery in a flame resistance bag, as an added safety measure, if the battery have been damaged in any way there is a risk it could combust during charging. This bag will prevent the damage to spread. Also great for storage.