Most factory car stereo systems are built as an afterthought to the car. The basic factory radio just can't keep up with the features and sound quality of an aftermarket receiver.
Upgrading to a well featured, high power car stereo is very economical. For less and $200 you can get iPod control, Bluetooth, HD Radio the ability to add a subwoofer and many other features!
These outputs send a signal to external amplifiers. A separate amplifier will sound better than the built-in amp, and play louder too. Most receivers offer at least one preamp output, but many have two or three.
Some receivers have a separate preamp output for a dedicated subwoofer amplifier. Adding a subwoofer is one of the most effective ways of improving overall sound quality in your vehicle. A dedicated subwoofer output from your receiver provides the optimal connection and best bass control.
You can connect a portable MP3 player or even a video source to some receivers. The most common type of connection is a mini-jack in the front, but some in-dash receivers have USB on the front also.
Much like your HDTV is better than your old cable. This just performs much better than traditional radio.
BETTER CD PLAYBACK
Most aftermarket CD players are able to track even badly scratched discs. Some feature electronic skip protection, allowing the player to read ahead on the CD to ensure skip-free playback
Some vehicles have a less-than-ideal stereo location which makes having a remote control a convenient bonus. Plus you can keep your eyes on the road while you make adjustments to your radio
An aftermarket receiver will usually produce more output power than your factory unit, but keep in mind that no receiver can match the power and sound quality of an outboard amplifier.
Most aftermarket receivers have a removable front panel. Take the faceplate with you, and your in-dash receiver is disabled. Anyone peering through your window will see only a plain blank panel or a flashing security light.