A hydratube is basically the top section of a tube with a perc in the neck.
But instead of a tube base or a beaker base with either a 45 or 90 degree joint, it has a downward facing joint.
It would be mostly useless for combustion, as your herb would just fall out if the bowl, but since a lot if vapes have a screen on both sides of the bud, it holds it in place, making the bottom a convinient place for the Gong joint.
I could rattle off a few vapes that it would work with, but tbh, it's probably be easier to come up with a list of vapes it wouldn't work with.
The use (for me) is as a convinient water tool that gives a nice moderate amount of filtration in a single chamber, and its also pretty portable (meaning it doesn't take up much room in a backpack or something, not for our and about use
)
Here's a vid of my hydratube with my solo, it might help you understand it's use