Because we can; for the fun of it; because it is usually pleasing to the eye; because it lets us use our creative talent - the list is endless, and creative energy is really not non-functional:-)

We decorate to experience beauty.

From Illusions, the Adventures of a Reluctant Messiah by Richard Bach:

"Richard, don't forget what you did today. It is easy to forget our
times of knowing, to think they've been dreams or old miracles, one time.
Nothing good is a miracle, nothing lovely is a dream. "

"The world is a dream, you say, and it's lovely, sometimes. Sunset. Clouds. Sky."

"No. The image is a dream. The beauty is real. Can you see the difference?"

We have a natural urge to bring forth Beauty, Strength and Wisdom into our lives... an evolutionary thing.

(this also explains the negative side which is to destroy that which is beautiful for no good reason, bullying and ridiculing those that acquire knowledge - by 'levelling' those that make us feel bad we experience a temporary feeling of superiority.)