A battery eliminator is a device providing power to an electronic circuit. It can be any number of devices, such as a direct current power supply, etc. to power an electronic device. But why we use a battery eliminator instead of using batteries?
The reason is because it can be a substitute if you do not have batteries, or you do not have the correct type, or if all of your batteries have run out of. Being that most electronic circuits run off of direct current power, you can use any direct current power source to power the electronic device, as long as you use the voltage it needs. So if all your batteries have died or you just currently don't have the right type, you can still power the device, using any device which can provide the direct current voltage which your device needs. Battery eliminators can do this.
Another reason is that if you're designing a circuit and you're trying to figure out the right voltage needed, constantly changing and swapping out batteries can be very tedious. With a battery eliminator, you can simply change direct current voltage just by adjusting a knob, and all you have to do is to rotate a switch.