What is added for delight rather than for function. Decoration is not waste — it is where a made thing says it was cared about.
Latin decoratio (an adorning, decoration), from decorare (to beautify, adorn), from decor (grace, beauty, from decere, to be fitting); what is added for delight rather than for function, marking that a made thing was cared for beyond mere necessity.
Its Latin root, decere, is about fittingness before it is about beauty — a decoration is judged by whether it belongs, not only by whether it pleases. What earns a place this way was made by someone who stayed past what the function required.