Capacitor banks are installed in the electrical room to improve the power factor which is related to the inductive load caused by motors etc .
The capacitors create a capacitive load which subtracts against the inductive load thereby improving the power factor.
Most power utilities have penalties when your power factor drops below a certain value.

Groups of large, specially constructed, low-inductance high-voltage capacitors (capacitor banks) are used to supply huge pulses of current for many pulsed power applications. These include electromagnetic forming, Marx generators, pulsed lasers (especially TEA lasers), pulse forming networks, radar, fusion research, and particle accelerators.