It's a line from Shakespeare, and correctly done it's "Mend your speech ..." As writers, we communicate on paper, but we still need to consider words, individually and collectively.
I used to teach high school, and often a student would object to my request that they clarify a sentence like "Mr. Jones saw several worms looking over his tomato plants." Invariably the cry would be, "But people know what I mean."
Possibly. But as a writer, your job is to make the job as easy as possible for the reader. He shouldn't have to stop reading to figure out what you just said. He shouldn't be pulled out of the story by the number of times you repeat a word or phrase. He shouldn't be lulled to sleep by sentences that all have the same cadence. And he should be given the best possible word to describe each item, the person, or situation.
Wordplay can be a delight when it's done well. Description can make you feel like it's you, not some character, who sees or feels what is happening. If you've ever had the experience of having to close a book for a while because it's just too intense, you know what I mean. If you've ever teared up while reading (or writing!) a passage, you know.
Although some of it comes from the reader's experience, a lot of the emotion of a book is wrought by good writing. Dean Koontz' Odd Thomas stories are told so beautifully that I don't mind that I don't believe in spooks, demons, and otherworldly visitations. Philippa Gregory's characters are so realistically drawn that I forget that I disagree one hundred per cent with her portrayals of Anne, Mary, and Elizabeth Tudor.
"The best words in the best order" (that's Coleridge on poetry, but it applies) bring results that engage the reader, but it takes work (and a thesaurus). Searching for the right word, the right phrase, takes time, thought, research, and oftentimes, many attempts. You search and search for just the right adjective or adverb, the exact noun or verb.
And oddly enough, you may find in the next edit that the best thing to do is to leave that part out altogether.