Serving ads to warm audiences - site visitors, cart abandoners, past purchasers - to convert intent you have already paid to create. It works: only 2-4% of visitors buy on first visit, and dedicated campaigns for high-intent segments like recent cart abandoners outperform leaving it all to ASC. But it is also the most flattering line in your ad account. ROAS loves retargeting because it claims credit for customers who were coming back anyway, so a team optimising for ROAS gravitates to retargeting existing buyers - easy wins that do not grow the business. Make nCAC the north star and retargeting stays a supporting act, not the show.
Benchmark. Cap ASC's existing-customer share at 10-30%, and lower it if nCAC is what you are optimising for (it should be). Cart abandoners from the last 7 days are the highest-priority segment; a ghost-audience holdout reveals retargeting's true incrementality fast.