Person trying to communicate: Ambiguous, vague, loose or metaphorical sentence
Pedant: Ah, I shall take that literally.
Communicator: You know what I mean.
Pedant: But you didn't say it!
You may well be the pedant in this exchange. I have been, often enough, and it's taken me a couple of years of linguistics courses to see why this is a silly position to take. (Alas, the preceding decades of Life were apparently insufficient for the lesson "Don't make other people's lives difficult for no reason".)
The answer to the pedant is, of course, that you did say it. If what mattered was certain choices of vocabulary and grammar, then speaking in flawless, metaphor-free Latin would give pedants no cause to complain - but no communication would have taken place. (Probably. If your pedant has a working knowledge of Latin, chat away.) If your words are sufficient for an exchange of meaning, then they have served their purpose. Language is not for grammar books, it is for communicating.
Now, whilst this is the view of pretty much everyone working in modern linguistics - and the de facto position of almost all humans - historically, philosophers disagreed, and their misconceptions led indirectly to modern pedantry.
Let us begin, as is often the case, with Aristotle, father of modern thought, wrong about almost everything. In this case, human communication.
His view goes like this: You have a thought. You put that thought into words. You speak the words. The listener hears the words. She converts the words into thoughts. You have successfully transmitted thoughts from your head into hers. Language, therefore, is just a code. At this point, obsession with the details of this code makes sense: if you say "Can you pass me the water?" instead of "May you pass me the water?", you have transmitted a different thought. You have failed to communicate.
But very clearly, this is not the be-all and end-all of human communication. What if you say "Can you pass me that?" and point? What if you don't say anything, but gesture frantically at the the jug whilst fanning your mouth?
Language is a code; it does assign meaning to otherwise arbitrary combinations of sounds or signs; but that code is a very sophisticated tool that we wield to help us with our underlying strategy: guessing what other people are thinking.
So whilst linguistics as a field does tend to be focused on how syntax (grammar), phonology (pronunciation) and semantics (meaning) work, there is a growing consensus that even with a complete understanding of language in its Ideal form, we won't truly understand it until we know how people use it. And that, ladies and gentleman, is pragmatics: the difference between an ideal sentence, and an utterance in its context.