when is it nessary to add a cap. to your system?
whats the real reason for a cap.
if needed,how do know what size cap to use?
whats a good brand?

if your lights dim it will help with that

A cap wo not help with dimming lights, that is not the purpose of them.

A capacitor is only good so far as the audio system is not trying to pull too much from the electrical system. A cap does not provide more power, it is designed to 'stiffen' the voltage to the amp, nothing else. If the current is not there, a cap wo not help.
The alternator must have at least 20% more amperage power than the entire vehicle & sound system combined for a cap to be of benefit which is ironically the same requirements for an amp to be efficient.

It is 0.5 Farads for every 500 watts of power.

Brand does not matter but u want an ESR that is less than 0.0016 ohms.

some/w the lights are pretty looking and it looks fancy,,,but unless your using 1000 watts RMS and up- it reall does nothing but help/w wiring neatness,,,

Think of the cap as a battery that can charge and discharge very quickly.
When the amp needs power, like when the base note hits, the cap supplies the amp with the extra power it needs.

25 years working in the electronics field,