voltage stabilizers use capacitor to store charge and same way regulate the flow of current in the electrical system. stabilizers tend to compensate for the minor loss in current flow due to
1- less charging by the alternator due to fluctuation in the rpm
2- aging of battery and charging capacity of alternator
since is maintains a constant flow of current, it somewhat contributes to better fuel burning due to sufficient charge by the spark plugs and car's other electrical components. u may also notice ur headlights are bit brighter. and overall electricals are working good. but that change to car's engine power is almost negligible. Ppl mainly use such stabilizers when they install sound system with heavy amplifiers etc.