Originally Posted by TheBlueMarvel

In the comics, the helmet has nothing to do with Hank's (nor Janet's, Scott's, Clint's, Bill's, or Cassie's) ability to alter their size it's the Pym particles that make that possible. The helmet allowed him to breathe in hostile environments, communicate via short-range radio waves, and allowed him to communicate and control ants. I'm not even sure that the movies established the helmet being used that way; I think it was the suit that regulated his height.