Those capacitors are used when you have a lagging phase angle caused by inductive circuits. The capacitors have a leading phase angle, which will cancel out the inductive lag.

I was taught this acronym: "ELI THE ICE MAN"
Current lags the voltage in an inductive circuit.

E=voltage L=inductance I=current

Current leads the voltage in a capacitave circuit.

I=current C=capacitance E=voltage

Luke Clarke
Electrical Planner for TVA.