monday.com’s formula column calculates one item at a time and cannot read another board. So there is no group subtotal, no SUMIF, and no lookup. Rollup adds those three things.
Total any column by board group, by status, by owner, by date — or by nothing at all for a single figure. Sum, average, minimum, maximum, median, counts, and distinct values, with each group’s share of the total.
Total only the rows that matter: won deals, unpaid invoices, work closing this quarter. Fifteen filter operators across text, numbers, dates, statuses, and people.
Aggregate across boards, either by following a Connect Boards column or by matching a shared key. Unlike a mirror column, Rollup can total the linked rows and filter them while doing it.