Car electrics are really noisy, and will have horrid spikes, surges and noise on them at all times when the engine is running. Even worse if the electrics are ageing, and truly horrid if made by the Prince of Darkness, AKA as Lucas. 400V spikes on the battery every plug firing, 30V pulses from the alternator and more as the ground wires typically are daisy chained to many devices inside the loom so they only need a single chassis point to connect during assembly.
Thus noisy, but putting something at the battery will do nothing for that, you need it on each device that is sensitive, to act on both the supply and ground wiring, which is why a radio set up is often supplied direct from the battery and has it;s own dedicatedfilter on the supply lead along with a short separate chassis connection to ground it, and often the higher power units have a separate battery for themselves only, fed from a separate alternator or from a charge controller on the regular alternator to split them electrically from each other.