In this case, you look at this and it's clear that the intent is to prevent you from backtracking. So ask yourself - why do they not want me to backtrack here?
And once you've asked yourself that, well, perhaps make an attempt to find out, if that happens to be a possibility.

Backtracking through the level, nothing seems to be off until you reach this area. Then the question of "why did they not want me to backtrack" seems to be answered.

Is that all there is to it?
Okay, in this case the answer initially appears to be "because the way the level is optimised means it was not designed to accomodate backtracking." However, I think you can ask yourself this question about a lot of design stuff and come up with more... design-oriented answers than just "because the game breaks" (although there's something to be said about how even answers like this would teach you some good tricks for masking issues like this.) And I think doing that is probably a good way to help improve design skills.
Even if your answer isn't correct it's probably still logical and in working in that sort of logical space you gain insight which you can then maybe apply if you ever design something in the future. Perhaps you will face the same problem to which their design decision was an answer or maybe it'll just be good practice for thinking through this kind of problem.
However, we can actually look a little deeper into the aforementioned example. Why does backtracking allow you to see this? They could have set up the level to not break when backtracking. Why didn't they? Because it would have been pointless. There's not really any reason for the player to backtrack here and it'd have been a waste of time to accomodate it. So, the easier solution is to just have a one-way drop to prevent casual players from backtracking and seeing this. Sure, it's still incredibly easy to do so with some classic Half-Life 2 box stacking but at that point the player is going out of their way to backtrack and probably shouldn't expect it to work properly.
So the answer to "Why can't you backtrack here?" isn't actually "because you'll see the edge of the world", it's actually because the level designer realised it was a lot saner to make the drop (mostly) one way instead of trying to build the level to work if the player decides to go backwards. Continuing to keep asking "why" is fun!