Plainly put: You can never truly arrive at or trust your own beliefs if you can't completely understand and articulate the best possible case against them. In order to do that, you must put yourself in the position of one who believes the opposite of what you do. Maybe you'll even find your own beliefs change in this process. But humility and courage of your conviction, if you have a conviction, demands that you can argue the other side better than anyone on the other side.
Also, where reasonable people disagree, there are always two versions of the truth. Learning to grapple with that fact teaches one not to see everything in pure black and white. Which has the benefit of teaching one to judge practices on the merits rather than individuals on their orthodoxy.