Adam Smith died more than two centuries before the advent of digital video, yet the Scottish economist’s invisible hand was on display in the market over the past year or so.

As the digital video market exploded, suddenly publishers found that they didn’t have enough ad space to sell. Then the market found a way: Outstream ads.

Since that time, the addition of such ads (which unlike pre-roll run independent of video content) has helped propel the market by offering new opportunities for ad buys. According to eMarketer, the U.S. digital video market is expected to nearly double by 2019.

Outstream may have seemed like a stopgap solution for the market. In fact, as an ad unit, outstream may be superior to pre-roll for several reasons:

It lets you advertise on text-heavy sites. There are lots of contextually relevant opportunities for video on sites that don’t contain video. For instance, if you sell hot water heaters, you would want to get your ad message in front of DIY blogs that might touch on hot water heater installation. Previously, you could only run an ad before a video relating to the topic. Outstream opens up new media opportunities to reach an audience that would have been unavailable before.

It lets your video advertising achieve scale and viewability. Targeting pre-roll greatly limits the potential number of consumers that you could target. For instance, the universe of people interested in hot water heater installations might be large, but only a certain percentage will be accessible via YouTube videos. With outstream, advertisers can consider a much larger pool of potential customers. Because such ads stop playing when they’re not being viewed, the unit also ensures viewability.

They help publishers monetize on mobile.  Outstream ads help monetize media consumption on mobile. Chris Pirrone, general manager of USA Today Sports Digital Properties, told The Wall Street Journal, that 70% of his audience consumes content on mobile. Many readers are linked from social media. “You may only get that reader for one story, so these type of ads help us make a little bit of money even if that person doesn’t come back again,” he said.

It’s a non-interruptive model. A preroll ad basically holds you hostage until you can watch the video you intended to view. In contrast, an outstream ad begins playing when it’s in your field of vision, but then pauses as you scroll down. In other words, it’s a non-interruptive model in which the viewer can choose to watch or not. In many cases, viewers may choose to watch an ad but decline to turn the sound on. A recent survey from ReelSEO found that 85% of Facebook videos for instance, are viewed without sound.

The non-interruptive nature of outstream advertising is also pertinent because consumers are increasingly fighting back against intrusive advertising. In particular, some 198 million global consumers are employing desktop-based ad blockers and another 408 million are using them on mobile, according to PageFair.

A confluence of factors, including the growth of video, monetization needs of publishers and the quest for a non-intrusive ad solution, are working to make outstream ads the de facto standard for video.