Perhaps I'm just in a long word frame of mind having recently dealt with pareidolia, being described as surreal and now euphemisms. They were brought into mind by a cracker I heard this morning. The process of changing someone's sex is not just a 'sex change operation' it is a 'gender reassignment surgery'. It sounds so gentle to be 'reassigned' BUT.....
A euphemism is a generally innocuous word or expression used in place of one that may be found offensive or suggest something unpleasant.
They can be used for three purposes.
To Soften an Expression
To Be Polite
Euphemisms to be Impolite
Of course it can lighten a passage of your writing by introducing a euphemism where the opportunity is suitable. In some respects the whole 'Innkeeper's' song in Les Miserables is an attempt to lighten the mood of the stage show and is perhaps more a metaphor but I'd like to think of it as a euphemism for greed and corruption.