Capacitors hold a charge which can be recalled for future use. It takes much energy to power some low frequency sub-woofers, in some cases so much that it over draws current from the amount that can be provided by the charging system of an automobile. For these type systems, if you were driving at night and your volume was high enough, it might dim your headlights when playing some BOOM-BOOM-BOOM low frequencies from your sub-woofer. On the other hand, if you have large capacitors installed as part of the audio system, it would draw power from them during this time to prevent your charging system from being overloaded.

they r made so that they hold energy and so that the alternater doesnt have to b forced into working more to recharge ur battery...

A capacitor makes a huge dent in your wallet, with minimal positive impact on your car's electrical system. Think of a capacitor as a small power surplus storage device. All it really does is catch a little overflow of current from the alternator, if there is any, and stores a limited amount of power internally. Considering that even a small cap can cost you over $100 plus install, they really don't offer much. If your audio system is higher in continuaous wattage (lets say for example 1000 W RMS total), you'd probably be told by an electronics retailer to purchase a 1 to 5 FARAD capacitor. A normal price for a 1 to 2 Farad cap can range from $150 to $300. Lets look at it from another perspective. You can get a decent deep cycle battery for around $150 to $200. These hold a much larger charge, and connected with a battery isolator ($25 on ebay), will give you a stand alone power supply for your sound system pulling minimal to nothing over the top of your vehicle's normal power consumption. So, your car's electrical charging system will last longer (likely longer than stock), your equipment will not affect essential functions like headlights at all, and wow, look at that, it costs less than a cap and does more for you. Aside from that, a cap will create more power draw on your car's recharging system if it is emptied. How can it be said to help relieve power draw, when it DRAWS POWER? If coupled with extra batteries and such, a capacitor can and will help in situations of extremely high current draw, but used as the sole solution to help with heavy power draws, they are not the best idea. Seems like a no brainer to me...