I’ve asked my son about the difference between 2.4 and 2.8 aperture.
The difference is the amount of light the camera lets in, and how “fast” the camera is. This means: on the iPhone X, slightly more light hits the lens, making the camera faster (shutter speed). The difference - just a third - is negligible, though. If you see an image taken with the iPhone 8 Plus and the same taken with the iPhone X next to each other, you wouldn’t really notice a difference.
But it will help slightly when in a darker area. The faster the lens, the better. That’s why Apple said the iPhone x will take better pictures in not-so-well lit locations.