If you flashed a new ROM, for example, your batterystats.bin folder still contains the old values from the previous ROM.
This can cause high battery drain, because battery is just "not sure" how many percent has your battery.
Battery calibrator formats this batterystats.bin file and deletes the values. From time to time, your device will automatically generate new values
based on the current ROM and current usage (therefore you can calibrate your phone not only when you flash a new ROM).
After a few days of using, you can notice the battery drain is less and less agressive and the battery life is much better than it was after flashing.
Give it a try!