No idea why it does, but it does, just give it a try and you will be amazed.
Not that you will be getting those "big milky" clouds, nothing like that nor that it will be anything like the 30% promised by those special (expensive) stems, but still, I found the improvement noticeable and it helps me getting a smooth, less harsh tasting vape, but like said: you have to take it the way you would drink a cocktail through a straw, slow and even.
Also the vape is much "cooler", probably because of the longer path it has to go.
The straw has less diameter, that might count as well.
I found that leaving some clean cotton inside the stem, against the 4 holes, collects a lot before getting down your stem and you have less trouble collecting it.
Doing that doesn't restrict the airflow at all. (untill it gets full)
After the cotton is full I remove it using the end of a knitting needle, put it inside the bowl and vape again.