|Mixing Amplifier MA-35 (Photo credit: Wikipedia)|
An amplifier is necessary for many musical instruments in order to connect them to speakers. Amplifiers also help produce unique sounds that you hear many musicians play, most notably guitar players.
Most guitar players can people with car stereos basically just want an amplifier to produce very loud sounds without a lot of distortion of the sound.
How good the amplifier depends on a number of specifications. The gain is the ratio between the output and the input and is usually measured in decibels.
Bandwidth is another important specification to consider. It is the range of frequencies that the amplifier can produce.
Another quality to consider it the efficiency, especially for car stereos. The efficiency tells you how much of the power that goes to the amplifier is actually applied to the output. Class A amplifier is the most inefficient, while Class AB amps are the best.
The bottom line is that people use amplifiers to make their cars and musical instruments sound very allow. More sophisticated musicians buy amplifiers to produce a higher and richer sound than what the instrument could produce just by itself.