well
, the purpose is related to bad things happening(distortion) with the waves mixing earlier and the length they have to travel and the bore material they are traveling through. Once they leave that bore there is no issue anymore and it is safe to mix. There are issues there depending on the construction/design and housing. Even the single driver CH9T uses a bore of brass with an aluminum sleeve to help get a smoother output. Mixing dual drivers can have even more issues. Not negated since once out of the bore the problems aren't an issue and can be mixed together. The cheaper Chi-Fi way is to put the armature or even duals right in the nozzle. Boom, no issue through the bore cause the armature output comes out at the end of the bore and is again free to mix after it. If you put the armature back further there is usually a small tube like here in the AX60 behind the screen.....which goes over the armature nozzle and keeps from mixing till after.
Not a gimmick but one of quite a ways to avoid issues that you will see being applied in earphones esp. multi drivers. Dangerous journey for those frequencies from the housing through the bore and into the ears. Gotta keep them safe and pure!