Big numbers feel reassuring. They're also easy to misread. Incremental reach is the discipline of asking: what did this channel add that wasn't already there?
Total reach vs incremental reach
Total reach is everyone a channel touched. Incremental reach is the slice of those people who weren't already exposed elsewhere. It's a smaller, harder, more useful number.
Why big platforms aren't always incremental
A platform can look enormous in isolation and still add little new audience. Once you can see overlap, the comparison stops being about size and starts being about contribution.