Ballasts are used for the ignition sequence of  high pressure sodium lamps, metal halide lamps and other condenser discharge lamps. Without ballasts the starting sequence will not be initiated. Electrical ballasts as well as electronical ballasts are connected in series to lamps and shouls always suit the lamps power.
Electrical ballasts reduce the use of electricity and enhhance the real power of the lamp by round about 5% (according to manufacturers approx. 5% more light emitting efficiency). Furthermore ballasts will not get hot and work almost without a sound.