I've run into a few instances, especially when on a cut, where I want to ignore or adjust the values for a given diary entry without affecting overall day/global settings.
One example that comes to mind would be net carbs. I'd like to be able to sometimes-ignore carbs like erythritol and allulose from certain diary entries, but not others. Also in this list would be things I'd consider to be "free" foods while dieting - I'd still like to track them in my food diary but not have them affect my overall calories/macros - things like lettuce, sugar free jello, low/no-sugar green veggies and so on..
If there were a set of toggles/overrides, but on a per-diary-item/as-needed basis, I would find that especially useful in scenarios like this.
Another instance can be general food items where I know something is off in the "closest" item that exists in the global food database... whether it's macronutrient(s), or micronutrients - sometimes I don't want to create a copy and adjust that way - it would be useful to have as-needed overrides to change individual values for a specific entry.