Residual alkalinity measures a water's buffering capacity specifically against the acidifying effect of dark, roasted malt — a more refined concept than bicarbonate alone, since it also accounts for how much calcium and magnesium are present to partially counteract that alkalinity rather than looking at bicarbonate in isolation.
Why the same bicarbonate number means different things at different calcium levels
Two water sources with identical bicarbonate readings but different calcium levels don't behave the same way in a mash — higher calcium partially offsets alkalinity's pH-raising effect, so residual alkalinity (which factors calcium and magnesium in alongside bicarbonate) predicts mash pH behavior more accurately than bicarbonate alone would. This is part of why Dublin's water, high in both bicarbonate and reasonably substantial calcium together, buffers a roasted-barley mash so effectively without needing much correction.
What this means for matching water to a target style
High residual alkalinity suits dark, roasted-malt-heavy styles well, since it counteracts the acidity that grain naturally brings; low residual alkalinity suits pale, delicate styles, since there's little alkalinity fighting against the mash's own naturally low acidity in that case. A brewer building toward a very pale lager from a high-residual-alkalinity water source typically needs an acid addition or a calcium-boosting salt to bring mash pH back down into range.