Most campaigns need the same idea in four or five shapes: 16:9, 1:1, 4:5, 9:16, and occasionally something a specific placement demands. Writing that down as a grid before production is the difference between composing once for all of them and re-composing five times.
The matrix also exposes duplication. Nine placements often turn out to need three actual pieces in three ratios, which is a substantially smaller job than nine pieces, and nobody notices until the grid is on a page.
It is deterministic work once the decisions are made, which makes the export side of it a script rather than a person.
Which ratio should be composed for first?
The tightest one, usually 9:16, with the wider ratios composed as extensions of it. Composing wide first and cropping down loses the subject; composing tight first and extending outward does not.
Should the matrix live in the brief or the delivery note?
The brief. Discovered at delivery it is a re-export at best and a re-shoot at worst.