They were originally for wire wheels with all the spoke nipples in the rim. Or, when there are conventional rivets holding the 2 piece steelies together. Anything that could rub a hole in the tube is a reason to use them.
It was recomended by mr snyder at snyders antique auto parts in oh that my father just go around the rim with duct tape 3 or 4 times. He did and no problems and it has 3 years . And snyders sells the liners.
I never had a problem on probably 50 or 60 rims over the years.. Now watch, since I said that I probably will have a rub through on my new project......
I would not use duct tape, it dries out and the edges get sharp and eventually cut the tube. The black plumbers tape works great (not the little black electrical stuff). If you have the liners I would use them. Also a good idea to sprinkle a little baby powder around the inside the tire first, so the tube can slightly move and properly center its self.
I have heard of guys using silicone caulk and laying down a good heavy layer in the center of the rim, enough to cover the ends of the spokes and protect the tubes.