Does the jacket of tubes used to cool the nozzle and combustion chamber in a rocket engine also give it some structural strength? Does it have any function relating to withstanding the pressure of the byproduct gases, or is it for cooling purposes only?
Take a look at this question: How are rocket nozzles made? In pictures there, you can see that the cooling jacket IS the nozzle in many cases.
In this picture I took of a flown, but scrapped SSME nozzle, you can see that the inside of the nozzle is constructed solely of the cooling tubes.
Here is a close-up.