Equalizers are built to control the loss and gain of frequencies within a sound system. This allows a sound system to sound natural and full. It also gives it the ability to maximize volume while eliminating feedback. Many stereos today are built with a graphic equalizer right in the system. But for a high-end stereo system this unit is generally separate and allows fine tuning. Since there are not usually any microphone inputs in a home stereo application, the adjustments usually do not have to take into account the ambient sound in the room, but do help compensate for the "acoustics" in the room.
Equalizers are frequently used in public address systems to sharpen the sound and reduce echoes. Stadiums, sports arenas and other venues will want a good sound system with a good equalizer.
Churches, with their unusually angled rooms and ceilings will especially benefit from having an equalizer in the sound system. As churches often have multiple microphones and speakers, a stereo equalizer is a must.
Bands, and other live traveling shows will perhaps find the equalizer most useful, as it is nearly impossible to construct a good sound system for every venue without adjusting for frequencies that will create feedback.
Most studios have an equalizer, as it is very useful for coordinating the various microphones and sound inputs into the system. It can also reduce and eliminate ambient noises like air-conditioners that may hum in the background.