In a 3D environment, it is easy to convey the notion of what a player can see and what they can't see. Depending on where they are looking, certain objects appear in their field of vision.

But in most 2D side-scrolling games, there is no sense of "vision". Generally you can see the whole map in front of you. Sometimes, you are afflicted with a "nerf" because you exploded a barrel of toxins, or a slime fell on your head, or you walked into a really dark room and your vision diminishes to just a small radius around you.

Here's how Hollow Knight treats this as an example:

As you can imagine, the character/player essentially becomes a light source, with the light intensity dropping off drastically to limit your vision around the map.

There are of course other ways game makers will limit your vision, usually with large objects or parts of the map, or large vacant spaces like this:

Again, while these are effective, they don't really replicate the same "loss of vision" in certain directions you'd get when you were playing a 3D game.

While there's no real "need" to mimic it, it does make for a fun mechanic to build a game around.

Now imagine the same types of hollow knight maps, but with how I'd propose we would treat vision.

You can see how because the character is facing the left of the screen, everything behind them is blacked off, because it doesn't make sense for them to be able to see anything behind them (assuming it were a 3D environment).

Imagine a game, be it an rpg, or a platformer, where you could only see the from where your character is to the edge of the screen you're facing, and everything behind you is blacked off similar to how the image above is. The only way to see what's behind you is to turn the other direction, but you'd never be able to see both sides.