The open fit doesn't stuff your ear canal shut with a molded piece. Rather the aid is behind the ear and the tube goes into the ear but doesn't close it off. You can hear low tones directly without going through the amplifier.

Because you aren't closing off the ear canal, it eliminates most of the problem with "feels like I'm talking in a barrel" "it echoes funny" etc

But if the hearing loss is bad enough, then that won't work.

As long as it works, I would many times rather have a behind the ear with an open fit. Did I mention behind the ear costs about half of an equivalent in the ear model?