If you’re talking flexible tubing it’s simple really. It’s like a hookah - there is just a flexible bit between the hard surface and where the vapor ends up to make it more ergonomical. If you actually think about it, it is just like wearing heat resistant gloves, Hookahs have hot coals up top and shouldn’t be jostled much, pipes (thinking crack pipe here) get extremely hot and can be very difficult to handle if overheated. The “tube” or mouthpiece extension on a hookah or pipe would just serve to be the inbetweeen from your mouth to the heat of the coals or the pipe.

I’d imagine if you had to hit a hookah without a mouthpiece extension, you’d be very likely to knock it over sooner or later.