Open world maps as checklists instead of game spaces

I wrote a lengthy blog late last year about the stagnant, "limbo"-like feel of most open world games' narratives. I had written that blog mostly before I played Metal Gear Solid V, and so I wasn't able to incorporate my thoughts regarding that game into the blog. But I did come to a new realization about open world gaming while I was playing MGSV. In my review of that game, I noted that:

"Even the open world itself feels constrained, as sheer cliffs prevent you from travelling too far off of the roads and serve to functionally railroad the player towards the small set-piece outposts and villages."
    - from my Metal Gear Solid V: the Phantom Pain review

I realized while playing MGSV that the game had built this large, open world (well, two large, open worlds really, but I hadn't gotten that far yet), but it didn't really care to let the player actually traverse that space or use it in any meaningful way other than scavenger hunting for collectibles. At least those collectibles felt relevant to gameplay though! Roughly half the map is dead space that the player can't even access. There was also this strange focus on using the helicopter to drop in and drop out of missions, rather than actually living in the game world, as the character had to do in Snake Eater. The map started to feel less and less like a place, and more like a convoluted mission-select screen. At first, this seemed like a strange, isolated example of an open world game that really doesn't want the player actually exploring its world. But as I thought about it, I realized that this isn't really a new phenomenon; it's actually just a very extreme example of what has become a sort of defacto state in most open world games.

The Afghanistan map of Metal Gear Solid V feels heavily constrained by sheer cliffs.

Think about it this way: in a linear game with rooms and corridors, every hallway and room should serve some purpose or function. In most games, this function will be some kind of skill or system mastery test. An action game like Devil May Cry will throw enemies at you to fight; a puzzle game like Portal will have a puzzle (or a piece of a puzzle) in the room to solve; a stealth game like Metal Gear Solid 3 will have a sneaking challenge or obstacle to pass; and so on. In the best games, each of these challenges will also provide a unique or novel test of skill or system mastery: unique combinations of enemies, unique puzzles, or novel arrangements of enemies and obstacles. Other games can use those rooms for thematic or narrative purposes. A survival horror game like Resident Evil or Silent Hill will usually put enemies, puzzle items, or supplies in a room, but some rooms might instead contain a scripted scare. In some cases, a room might even be left completely empty in order to build some kind of tension or anticipation, or to delay the release of already-built tension or anticipation.

So what is the gameplay purpose of an open world map? ...