Amazon's advertising business, once a niche part of the company's sprawling empire, is now big enough to merit its own conference.

Last week in Seattle, Amazon attracted roughly 400 people to "AdCon 2019," CNBC has learned. The inaugural two-day event featured the company's most senior ad executives and sessions to educate businesses on "how to use Amazon Advertising to create connections with shoppers at key moments across the purchase journey," according to a page for the conference on events site Cvent.

Amazon's ad unit, which makes money by charging brands to promote their products across Amazon properties in a variety of ways, topped $10 billion in annual revenue last year, and eMarketer predicts that, in 2020, it will account for 10% of U.S. digital ad spending. That puts Amazon third, behind only Google and Facebook, which control a combined 60% of the market.

More than being a revenue driver, advertising is a major profit center for Amazon. While Amazon doesn't currently disclose its profitability, Piper Jaffray estimates the ad business will surpass Amazon Web Services in 2021 as the company's main profit engine.

The invite-only conference included case studies from brands like mattress company Tuft & Needle and pet food provider I and Love and You. There were recommendations for reaching new customers, techniques for using sponsored ads and breakout sessions on best practices for hardlines (electronics, appliances), softlines (clothing, linens) and consumables.

"We will share learnings and best practices that will help you plan and execute to achieve your business objectives," the site said.