They give a lagging power factor (which is generally undesirable), but that's not their purpose. The purpose of the inductor is to provide current regulation (limiting) to the tube and also to help it start, the power factor influence is just a (negative) side effect.
Inductive ballasts are used to limit the current through a gas filled lamp once it has fired.
Without them, the current in the lamp would become very high and possibly damage the lamp.
However the current in an inductor lags behind the voltage across it, so it has a lagging power factor.
This reactive current can be reduced by the use of a suitable bypass capacitor across the input to the lamp. Doing this reduces the current that must be supplied from the mains power supply and ultimately saves fuel at the power station.
It does not reduce your power bill if you are a domestic comsumer, because you would not pay for reactive current. Commercial users can be required to reduce their power factor.