Their primary function is as a profit tool for the retailer.
They don't do anything audibly.
Many believe they decrease the load on the alternator which is simply not true. They are a storage device much like a battery. If the alternator is undersized simply adding more storage is not going to help anything.
In some cases they can reduce minor headlight dimming but that is not at all a guarantee and only serves to cover up the fact that the alt might not be cutting it.