A discharger is critical for any AEG since you should always discharge your batteries fully before recharging. This ensures that the battery is able to achieve a full charge each time. Failure to discharge partially used batteries before recharging will lower the life expectancy and capacity of a battery so this is a must buy. If you opt to purchase a charger that has a built-in discharger, then you do not need to purchase this discharger separately.
High performing discharger will rapidly empty your batteries for a cleaner recharging. Remember that NiCD batteries will always need to be discharged before being recharged. NiMH batteries do not necessarily have to be discharged every time but you will get better charges if you do.