What's the benefit of using audience expansion?

Audience expansion reaches additional users who are similar to your selected audience segments. You define the audiences you believe are right, and expansion then finds more people who resemble them and are likely to respond in the same way, which lifts reach beyond the size of the original segments without abandoning the targeting logic behind them. That matters because manually defined audiences are usually too small to sustain a campaign at scale, and because the people who convert often do not match the segment definitions an advertiser would have thought to pick. Control is retained: the advertiser sets the degree of expansion, exclusions still apply, and performance can be monitored to check the expanded reach is converting rather than just adding impressions.