There are a few main benefits from adding cheap car amplifiers to your system:

Higher sound quality: When you crank the volume up on your dash, amplifiers take the role of delivering sufficient power to your speakers. Because the power from the amplifier is clean and plentiful, you will expect little or no distortion from your speakers. An amplifier will allow your speakers to perform at top levels, delivering powerfully loud sound without lacking clarity.

Your in-dash receiver would struggle to provide efficient power for your upgraded speakers at higher volume levels, thus creating a distorted sound.

Power for subwoofers: No matter what size subwoofer you're going to use, it's going to need an amplifier to generate enough watts in order for the subwoofer to produce enough bass waves you can actually hear and feel. Using a subwoofer without an external amplifier is equivalent to not having a subwoofer.

Matching Amplifiers With Speakers
Most cheap car amplifiers can perform multiple tasks for different applications, or set-ups, whether it's for speakers or subwoofers.

Full-range and component speakers: Cheap 2 channel car amplifiers can be used if you're looking to only power a pair of speakers. If you have 4 speakers, a 4 channel amplifier will be a perfect match. Be sure the amplifier has a high pass filters so the mid, and most importantly, high frequencies can reach the speakers.

The most efficient way to power a subwoofer or pair is by using cheap 1 channel car amplifiers, also known as mono class D car amplifiers. A mono amplifier will deliver brute force while running cooler and draining less power from your car battery. They also have a bass boost, built-in low pass filters and are designed specifically to only reproduce low frequencies.

You can also use a 2 channel car amplifier to power two subwoofers or bridge two channels into one and power one subwoofer. Usually when bridging two channels into one, you'll generate more power from the amplifier than it could have generated in 2 channel mode.

Speakers and subwoofers: Cheap 4 channel car amplifiers will do the job here. You can use two channels to power your front speakers and bridge the other two channels together and power a subwoofer. A 5 or 6 channel car amplifier will open up even more options.

Matching Amp Output To Speaker Input
This is where you need to take care. If your speakers or subwoofers can handle 150 watts RMS (continuous) power, try your best to deliver almost the same amount of watts from each channel of the amplifier. This will allow not only your amplifier to perform it's best, but also your speakers or subwoofers.

For example: If you want to power 4 speakers that handle 100 watts RMS each, match it with a 4 channel amplifier that produces 100 watts RMS per channel.

Overpowering as well as under powering your speakers and subwoofers can cause damage over time. Also, if cheap car amplifiers are being forced to provide more watts that they can actually produce for a subwoofer(s) and/or speakers that demand more power it cannot achieve, it may also cause damage to the amplifier. The "stress" will simply cause the amplifier to shut down or turn off due to overheating from over working itself.

Go back to Car Audio - General Overview.

Or, learn more about:
Car CD Players
Car Speakers
Car Subwoofers