In a DND game, even if the players can't die, they can still fail. The country could be ruined, the world could be invaded, the players could simply not accomplish what they set out to achieve. That to me is the critical aspect, I think the main one Reynard was highlighting.
Success or failure is only a single aspect of the subject. Overall, whether in combat or out, whether in character generation or adventure design, the uncertainty element adds to the game.
Let's say the players decide to have their characters ander into the wilderness in search of a few random encounters to get those last couple hundred XPs to achieve their next level. Obligingly, the DM pulls out his handy-dandy "Forest of Blight" random encounter chart, which is not a level dependent list of possible encounters but a "simulationist" list of things that live in the forest, from lowly goblin bandits to a huge plague dragon. He rolls "ogres", which we'll say is more or less appropriate for the party, turns to the MM and rolls 200 of them. Thinking fast, he decides they have to have a place to live and opens up to the random dungeon generator and asks the players to take a short smoke break. Ten minutes later, he's got a small ogre fortress and a treasure hoard containing a holy avenger (rolled randomly).
Suddenly, the campaign includes an adventure about a small army of ogres who killed a legendary paladin who sought to defeat them, only a day's march from the campaign's home town. It's not an adventure or setting element the DM would have come up with on his own -- he had other plans -- but such a thing would automatically enhance the game and the campaign world.