A car amplifier acts as the power source to your sound system. Technically, the amplifier modulates power generated from the car battery. An amp specified to your speaker's inputs will fully orchestrate your sound system with low bass beats and high frequency sounds.
This amplification process is a necessary part of every single home and car audio system, and the power of an amp dictates how loud and distortion-free that the sound will be.
Every system has at least one amp, even if it's built-in to a head unit, and some even include more than one. For instance, it's fairly common to include a dedicated car audio amplifier to drive a subwoofer.