Why does supplier engagement matter in sustainability reporting?

Standards including ESRS E1 and the GHG Protocol Scope 3 Standard ask what share of a Scope 3 total rests on primary data from the value chain, which makes supplier engagement a reporting requirement rather than a nice to have. The practical problem is volume: a long tail of suppliers, each asked by many customers. Prepopulating what is already public, and asking only where a primary figure changes the total, is what makes it tractable.