Brittany Hennessy, senior director of influencer strategy for Hearst digital brands, says influencers are an important tool for companies that advertise.

She says consumers look to influencers for trusted and peer-like recommendations, something they don't often get from traditional ads.

Influencer marketing is projected to be worth between $5 and $10 billion by 2020, according to Hennessy.

Whether you're aware of it or not, it's likely that most things you buy you probably came across on social media — just think about the last time you were scrolling through Instagram and saw a cool hotel, place to eat, piece of art, or article of clothing. It was probably posted by an influencer.

Companies are spending millions of dollars paying people with large social media followings to promote their brand, also known as influencer marketing, because consumers look to their peers for trusted recommendations, something they're not getting from traditional ads, according to Brittany Hennessy, senior director of influencer strategy and talent partnerships at Hearst Digital Media.

Hennessy scouts and manages influencers for brands including Cosmopolitan, Esquire, and Harper's Bazaar. She's also the author of "Influencer: Building Your Personal Brand in the Age of Social Media."

"[Influencer marketing] is a business that's projected to be worth between $5 billion and $10 billion by 2020. That's an insane amount of money brands are spending. They're spending more than they've ever spent on influencer marketing and the number's only going to get bigger," Hennessy told Business Insider.

The reason influencers are such powerful advertisers is because they're regular people, she said.

"It's people that are the target demographic talking to the target demographic. And that's what makes influencer content so much different than something a brand would make," she said. "Nobody's blocking influencers — they're opting-in to that. We're now in the land and era of opt-in. And people opt-in to follow these influencers because they like the content."

Read more: A woman who pays Instagrammers and YouTubers for brands like Cosmo and Esquire explains why working with influencers is 'almost guaranteed' to increase sales in a way TV and print ads don't

And not only do they like the content, they're often buying whatever is being promoted.

"You post your piece, it may get whatever percent engagement rate, you post something similar from an influencer, it's twice as much," Hennessy said. She said brands have a stagnant view of how a product should be shown in advertisements, but an influencer can make it more appealing to the average customer.

"Now people are buying that shirt because of the way [an influencer] styled it," Hennessy said.

Instagram rolled out a touch-to-buy "shoppable" feature in October 2017. The feature has analytics showing how many people view product information and click through product pages after being redirected from the social media app, according to Tech Crunch. In May of this year, Instagram announced users can register credit card information to make native purchases directly from the app, according to Tech Crunch.

While influencers are a form of advertising — they're paid by companies to sell a product, after all — they're not as stale, and that's why they work, says Hennessy.

"In a perfect world, you should not be able to tell what was an ad and what wasn't and that's the whole point," she said, adding that influencer marketing is here to stay.

"It's totally going to stick, people care so either you're going to learn about it now and you're already late because ... this industry is almost a decade old already," she said. "Again, were not saving lives, we're not performing surgery on anybody, it's advertising."