it is reasonable to assume that superstitions which caused real fear called for the creation of euphemisms long before the need to describe things in their various aspects or subtle shades caused the appearance of other synonyms.
The Christian religion also made certain words taboo. The proverb Speak of the devil and he will appear must have been used and taken quite literally when it was first used, and the fear of calling the devil by name was certainly inherited from ancient superstitious beliefs. So, the word devil became taboo, and a number of euphemisms were substitutes for it: the Prince of Darkness, the black one, the evil one, dickens (coll.), deuce (coll.), (Old) Nick (coll.).
The word God, due to other considerations, also had a great number of substitutes which can still be traced in such phrases as Good Lord!, By Heavens/, Good Heavens!, (My) goodness!, (My) goodness gracious!, Gracious me!
Even in our modern emancipated times, old superstitious fears still lurk behind words associated with death and fatal diseases. People are not superstitious nowadays and yet they are surprisingly reluctant to use the verb to die which has a long chain of both solemn and humorous substitutes. E. g. to pass away, to be taken, to breathe one's last, to depart this life, to close one's eyes, to yield (give) up the ghost, to go the way of all flesh, to go West (sl.), to kick off (sl.), to check out (sl.), to kick the bucket (sl.), to take a ride (sl.), to hop the twig (sl.), to join the majority (sl.).
The slang substitutes seem to lack any proper respect, but the joke is a sort of cover for the same old fear: speak of death and who knows what may happen.
Mental diseases also cause the frequent use of euphemisms.
A mad person may be described as insane, mentally unstable, unbalanced, unhinged, not (quite) right