A tagline is what you say once positioning is decided. Most briefs skip the decision and go straight to the sentence — which is why so many taglines could sit on a competitor's homepage without anyone noticing.
Positioning is a choice about what you're willing to not be. Every category leader we've studied made that trade explicitly: the discount airline that refuses to be comfortable, the consultancy that refuses to be cheap. The tagline just reports the decision.
So the brief we start with is never 'what should the tagline say.' It's 'what are we willing to give up to be unmistakable.' The words come after, and they come fast — because there's finally something specific to say.