I'm not sure which definition you're referring to, but every one I've seen you'd be hard-pressed to use left as the 'good' value. The type's definition favours 'right' for Functor/Applicative/Monad/etc instances.
There are zero implementations of Left being the Ok value, just this hypothetical put forward by another commenter.
edit: I should specify that I mean when Either is used as a 'Result' type. There are plenty of other use cases you can use Either outside of Ok/Err values.