Lured by the prospect of greater reach with fewer resources, retailers are embracing programmatic ad buying. In doing so, they’re joining the ranks of a growing number of marketers choosing to leave the buying and selling of online ad placements up to technology and data-driven bidding that’s guided by the notion that software and machines are more efficient, effective ad placers than people.

These retailers are part of the growing shift toward programmatic, or automated, advertising—a process in which algorithms, informed by consumer data and parameters set by the advertiser, dictate when and where online ads will be the most effective. A retailer sets the budget, audience and goals it wants to meet, and then a software system processes that information. All of this takes place in a behind-the-scenes auction within milliseconds while a page loads.

U.S. advertisers have more than doubled their programmatic ad spend since 2014, according to research firm eMarketer Inc. Programmatic digital display ad spending reached $25.23 billion in 2016, up from $11.59 billion in 2014. eMarketer projects it to climb to $37.88 billion by next year.