It's not about sending email. They hash the normalized email and use the hash to track activity across websites and services to build user profiles for real-time ad bids.
Facebook (of all places!) is one of the few companies that makes this transparent.
If you have a Facebook account, and live in GDPR-land, go check which companies illegally passed on your data (https://www.facebook.com/adpreferences/ad_settings -> Audience based advertising) and report them!