When you map each source directly to each destination, the work grows as the product of the two. Three sources feeding three destinations is nine maps to write and maintain; add one of either and you are rewriting a column of the matrix. Worse, a change to one partner’s format ripples into every map that touched it.

Route everything through a canonical message instead and the mesh collapses into a hub. Each source needs exactly one inbound map (its format → canonical) and each destination one outbound map (canonical → its format). Three plus three maps, not nine. A new source is one new map; it can immediately reach every existing destination because they already speak canonical.

The canonical message is a deliberate internal contract — see the canonical data model pattern for how to design one per business entity. The rule here is simpler: nothing on the bus is ever in a partner’s native shape. External formats live only at the edges, which is exactly where mapping belongs.