In level-based games that permit the possibility of back tracking, it is not uncommon to find hidden challenges that are, at the first moment the player encounters them, insurmountable. For example, there could be a rock glowing a strange hue that is blocking the way to what one knows to be hidden treasure, but try as they might none of their current abilities are able to move it. In a later level in the game, however, a new power, party member, item, or some other element is introduced that allows the player to move gigantic boulders. They remember that this ability would have come in mighty handy in earlier levels, and replay those stages in order to reveal secrets.
This helps the replayability of the game, as the state of play always feels as if it were evolving, keeping the player interested. It also introduces a new kind of play style, that of the achiever. Some people feel most engaged in the pursuit of completing a game 100%. That means finishing all tasks/quests, collecting all items, unlocking all areas/characters, and so on. This kind of checklist-style endeavor makes a lot of players feel very accomplished, and they regard the game favorably as it provided them with a sense of fulfillment.
Example: LEGO Games
In all LEGO games, more characters are unlocked as the story progresses. Each character has individual traits and abilities that make them better (or worse) at accomplishing specific tasks. Upon the first play of every level, the game provides the player with all the characters they will need to clear the stage, but not necessarily the ones that will gain them access to every extraneous portion of the level. It is often that upon completion of the narrative, once all the core characters are unlocked, a player will pass through the game one more time in order to unlock everything that was missed the first time around. This can add many extra hours of gameplay on top of the initial playthrough, and that 100% platinum trophy may feel even more rewarding than completing the story did.