The cap takes the strain off the battery. Heres a example for a car.

Battery - Cap - Amp - Speakers

That's how most people do it. Now for my setup when I had the car running and before my carputer was stolen.

Battery - cap - carputer - Amp - speakers

The battery charges the cap and from the cap it goes to what ever device you connect it to that uses high amps and wattage.
To live is to let die