Why do speakers who give persuasive speeches often use connotation?

Using different words gives readers/listeners a different feeling. If I were giving you a speech about dress codes, trying to persuade you into thinking they were good, I wouldn't use a sentence like, "Dress codes mean lazier mornings, because it takes away the task of figuring out what you're going to wear." 
I would replace lazier with the word relaxed or simple. 

His writing and speech is quite different than today’s. students struggle understanding because of unfamiliar words.