How does EA benefit from Game Changers?

In my opinion, it’s beneficial both ways. EA gives the content creators access to content, which the Game Changer uses to promote the game for EA.
This isn’t a requirement, being a Game Changer doesn’t mean you have to create content with what they give you.
For content creators like custom content (CC) and mod creators, it’s handy that they can test their mods and CC early on the content given by EA.
For me personally, I enjoy that I can test a game before I help people with it. Before becoming a Game Changer I had to quickly go through a pack like Jungle Adventure to find an answer. Spoiling most of the pack for me.
Having a closer connection also helps with passing along feedback and to ask and learn things about the game.