It doesn't need to help us resolve the disagreement to be useful to explain why the disagreement is there in the first place, though.
And, in practice, there are large groups of people who do share the broad definitions of "fairness", and therefore saying that something is fair or unfair is useful to communicate the idea within those groups. This is not something that people really like to see put quite so explicitly, but when we say something "this should not be so because it is unfair", it carries an implicit "... and I don't care what those who disagree with me about what 'unfair' means think".
And, in practice, there are large groups of people who do share the broad definitions of "fairness", and therefore saying that something is fair or unfair is useful to communicate the idea within those groups. This is not something that people really like to see put quite so explicitly, but when we say something "this should not be so because it is unfair", it carries an implicit "... and I don't care what those who disagree with me about what 'unfair' means think".