If AI can help an advertiser get the outcome they want in 1000 ad impressions instead of 5000 ad impressions (with more precise targeting or more appealing creative), Meta can 5x the price of those 1000 ad impressions, keep the advertiser happy with the same ROI they had before, and sell 4000 additional impressions to someone else as incremental profit.

Or they could 4.9x the price to show the current advertiser a better ROI and encourage higher spend from them, while still having the 4000 extra impressions to sell.