Sometimes, you just need to crank up the volume. Car audio systems must overcome a excellent deal of ambient noise--a vehicle traveling down the highway in the community of other loud engines is surrounded by a formidable background babel level. Factory-installed head units have a modest expanse of amplification (generally about 10 watts per channel or less), and myriad aftermarket head units have at least twice that.
You could consider amplifiers with one, two, or four Car Audio channels, but you'll itch a channel for each speaker; this early increases the complexity and price of the system. Also, if you'll be adding a subwoofer for killer low-ends sound, you'll usually demand a cut apart amplifier. Remember, every speaker will custom one ditch of amplification. Four-channel amps are, by far, the most popular.