It's not only to calibrate things (which you can really only do by watching the battery terminal voltage change - there's an app for that -
Battery Calibration (Root) [but the phone has to be rooted]), it's used to condition a battery that hasn't been used in a few months. Charge it fully (about 30 minutes past 100%), then use it until the phone tells you to recharge it (or the phone shuts off). Do that 3 times. (Don't rush it - if you don't use the phone much, it could take 3-4 days to discharge.)