Confluence is the guarantee that different Reduction paths reach the same result.
A Term may admit more than one possible Reduction step — more than one place where a Function can be applied to its argument. Confluence guarantees that the order in which these steps are taken does not matter: all paths converge to the same Value.
Confluence is forced by the algebra of Judgements. If different Reduction paths could reach different results, Judgements about Terms would be ambiguous — a Term could have one property along one reduction path and a different property along another. The coherence of the Order on Judgements demands that evaluation be deterministic in outcome, even if not in path.
Confluence echoes the commutation of Flow and Nucleus later in the derivation: there too, it does not matter whether you first transform and then consolidate, or first consolidate and then transform. The principle that order of operations does not change the result recurs across levels.