For safety reasons. If there is no connection to the line, e.g. by means of a power supply, there is no part that could break and do any harm. Great care has been taken that the usage and handling of the batteries is as convenient for you as possible. First, you only need standard batteries. No special and expensive batteries. Second, the unit consumes very little power so that the EEG NeuroAmp will run for up to 100 hours on one battery set or charge. We recommend using rechargeable batteries in conjunction with a standard automatic charger that always contains a charged set of rechargeable AA batteries.