The fact that more publishers are adopting header bidding tecnology is allowing DSPs like Mediamath to identify more users via programmatic.
The incremental reach allows advertisers to identify more new users to the pool of known users (retargeting) and allows also advertisers to reach them more often.
The header bidding has then benefits for buyers. Because until now the known benefits were known for publishers: before the header bidding the auction was happening on the exchange level. With header bidding, many exchanges dispute the bidding on publisher level. This allows the publisher go to several auctions and that means more revenue.