Vital (adj; from latin vita, life)
Full of life
Necessary to continued existence
Recording data regarding life
Applied to living cell or tissue
Bolded one is the definition when you're talking about a vital feature.
Please, let's not use words where they don't belong. A "vital feature" would be the ability to move. I agree that being able to hide all guideline markers would be fun, but I don't think you can state that without this feature, the game cannot be played by you.