Using header tags, or header bidding, to replace the waterfall placement of managing feed partners is a powerful way to boost remnant ad space revenue by getting advertisers to compete in a truer auction over your impressions.
In essence header bidding moves the auction from a second price auction (whereby the winning buyer wins, but only pays a penny above the next highest bid), to a first price auction, whereby the winner pays their bid price. It also maximizes revenue through not having to guess about the eCPM of each fill partner. While partner A may average an eCPM of $3 and partner B averages $4, there are impressions (and users) partner A would pay a lot more for. If they never get a chance to see that user, do to a waterfall set-up, your revenue as a publisher is getting hurt.
Here’s a good intro article on the subject of header bidding through header tags.