Most car radios come with built-in amplifiers. However, this amplifier is usually quite tiny. Consequently, they can only deliver limited amplification. Most audiophiles will purchase an amplifier unit separately to get better amplification.
The main reason you would need an amplifier is if you upgraded your speakers. Consequently, the built-in amplifier is just not strong enough. If you fail to do that, you may not get the full benefits that come with upgraded speakers. Additionally, if you install an external sub, you have to install an amplifier. Failure to do that will result in some poor audio quality. Subwoofers usually consume quite a lot of power. In most cases, an amplifier has little influence on sound quality. That is dependent on factors such as the quality of the input devices such as the DVD player. However, a low-quality amplifier may create problems for the sound quality in the vehicle. For one, it may have low-quality crossover capabilities. It separates low-frequency and high-frequency audio. A good quality amplifier for your car audio will cost $150 to $250. There are also units that cost $20 to $50 that can deliver good quality.