That is, if the mechanism designer wants to implement some outcome or property, they can restrict their search to mechanisms in which agents are willing to reveal their private information to the mechanism designer that has that outcome or property. If no such direct and truthful mechanism exists, no mechanism can implement this outcome/property by contraposition.
In mechanism design, the revelation principle is of utmost importance in finding solutions. The researcher need only look at the set of equilibria characterized by incentive compatibility.