Accessory:
The word accessory is French; it is something additional, not necessary, that still contributes something good to something that is necessary. Accessory is a very broad concept, both in the automotive industry and in terms of everyday needs and fashion.
For cars, for example, extra spoilers, a multifunctional radio, seat heating, etc. can be classified under the term accessory.
Without these essential accessories the car would still function well, but with them it is just a bit nicer. That is what is meant by the first sentence.