Aristotle listed four reasons that rhetoric is useful. The 3rd reason is quoted below.
“[W]e must be able to employ persuasion, just as strict reasoning can be employed, on opposite sides of a question, not in order that we may in practice employ it in both ways (for we must not make people believe what is wrong), but in order that we may see clearly what the facts are, and that, if another man argues unfairly, we on our part may be able to confute him.”
Aristotle goes on to say that the true side of the argument will always be easier to argue.
“Nevertheless, the underlying facts do not lend themselves equally well to the contrary views. No; things that are true and things that are better are, by their nature, practically always easier to prove and easier to believe in.”
Quotes are from The Rhetoric and Poetics of Aristotle, p. 23 (from The Modern Library College Editions)