The ability to focus individually a layer (world layer, player layer) has its benefits for two reasons.

Depth error - depth of field uses (obviously) the depth buffer. This leads to ghosting around edges of objects and can make photographs appear "smudgy" or emphasize aliasing issues.

Focusing the camera takes effort - wouldn't it be cool if you could just take a selfie and have the focus work blurring the background every single time without needing to click a bunch of sliders to alleviate the issues mentioned above? ^^