You know what's fun? Video games. You know what's not fun? Being forced to replay a part of the game you've already beaten. That's why I wish game designers would stop making us do it.
Dark Souls is a game rife with retraversal: most of the boss fights don't have a bonfire (respawn point) near the entrance, so when you die at a boss you go a long way back and must fight your way through a bunch of normal enemies you've already killed before you can try the boss again. Either that or try to run past the mooks, which is often even more dangerous. And while Dark Souls may be an extreme example, it's by no means the only game to make the player replay sections.
I think part of the reason it's so prominent is because designers look at games like Jedi Knight: Jedi Academy, where you can save literally whenever you want, and see that it causes problems: you can cheat your way through any fight by saving every time you hit and don't get hit back. While the player can self-impose a restriction against doing that mid-fight, the game's lack of any mention of this potential for abuse leads you to believe that it's the way you're supposed to play, and so a lot of players do it. (Even I did.) Players should not have to house rule a video game to make it work. Designers think, as a result, that we have to find a balance between letting the player do this and going too far back when we die.
But there's a clear line between saving during sections and between sections. It's not just a difference of degree. A "section" is a self-contained challenge - that is, you start it with full status, and when you finish it, your status is restored. Final Fantasy 13 implements sections and the save system perfectly. Your party is automatically healed between fights, and when you die, you go right back to the field just before you entered the fight, not only preserving your progress in the area but also allowing you to reconfigure your team, equipment, or paradigm deck before trying again, or even give up on the fight and go somewhere else. But you still have to drain the entire enemy group's health bars in one try, so it's not abusable.
Not that every game has to treat individual fights like that. In Call of Duty: Finest Hour Update: I recently found out that this game doesn't work the way I remember. You lose unused health packs when you go back to a checkpoint. But I couldn't think of a better example I've played, so just imagine for this point that you don't. , your health doesn't regenerate after each encounter with an enemy, and it doesn't save after each one either. This isn't unfair because the game is balanced so that losing your entire health bar in one encounter is not a credible threat. Instead, they expanded the idea of a "section" to encompass an entire level, or at least a significant portion of it. The challenge of Finest Hour is not simply to win each encounter (which would be trivial), but to do so while taking as little damage as possible, so that you can make it to the next checkpoint with your limited health bar. This is also a legitimate way to design action games, provided the checkpoints aren't too far apart.
It might sound like I just contradicted the shit out of myself. I said that retraversal being bad is not a matter of degree, and then said that COD's progress-saving model is legitimate "provided the checkpoints aren't too far apart". Sections should always have a save before and after them; but when a section can contain multiple encounters, dividing the game into sections appropriately is a matter of degree. The reason this is a valid excuse for COD but not for Dark Souls is because Dark Souls does this with boss fights. Boss fights should always be their own sections; it ruins the feeling of grandeur if they're not. It's like if the the last two chapters in a novel didn't have a divide between them. If the motivation is that your boss isn't hard enough to be a self-contained challenge, then make the boss harder. (Most Dark Souls bosses are actually plenty hard enough to be self-contained challenges.) Another aspect is that a lot of the retraversal isn't even gameplay, it's just running through an empty area (looking at you Ornstein & Smough).