Ampere reports the charging and discharging current of an Android device in real time, alongside voltage and battery temperature. It answers a question that is otherwise difficult to answer at all, which is whether a particular charger and cable combination is delivering what it should.
The practical use is comparison rather than absolute measurement. Connecting several cables in turn to the same charger and reading the current for each identifies the poor one immediately, and a cable with thin conductors or a degraded connector will show a substantially lower figure than a good one on the same source. The same method identifies a charger that negotiates a lower power level than expected, which is common with cables that lack the marking a source requires before it will supply higher current. Discharge readings are useful in the other direction, since observing the current drawn while the screen is off reveals whether something is keeping the device awake, and a device that should be drawing very little while idle and is not has a background process worth finding. The readings come from the operating system rather than from dedicated hardware, so accuracy varies between devices and manufacturers, which matters for comparing two phones and does not matter for comparing two cables on one phone.
It measures rather than diagnoses, so interpreting the numbers is left to the user. That is appropriate for the audience and makes it less useful to someone who wants to be told what is wrong.
Link: Ampere on Google Play