What is the advantage of kWh based tariff?
The advantage of billing in kWh is that it reflects actual (real) power used by the consumer. Of the alternatives (kVA, Ah, etc.) it's the most fair.
Large industrial users with reactive loads pay a penalty if their power factors are too low, so this encourages them to invest in power-factor correction at their infeed side.
Lately, this is also the case in consumer: switchmode power supplies of 75W or more need to have power factor correction in them (EU Directive EN61000-3-2).
Related question: Why is electricity consumption billed in watts and not amperes?