>>387377
They're basically disposable masturbators, that are only a couple inches deep, so it's less than you penetrate it and more that you stretch it and rub it over your dick. Typically comes with a tiny packet of lube for a single use, supposed to be thrown away after that single use. They're made of such weak material that even if you carefully clean it (difficult because they're closed-ended) it won't last more than a few times. So if you buy a penetrable that is intended to last, and use it more than like ten times, you're already getting more use out of it for the price than for tenga eggs.
They come in a grand selection of fancy colors and internal textures (for all the little your dick can discern them) if you wanna play Pokemon and collect 'em all.
Really, it's purely cultural. Japanese are very sexually repressed people, because they have to hide all their perversions (including using sex toys) from everyone or face social suicide. So for them it makes sense to have a toy that is used only once in a bathroom, and then the "evidence" immediately disposed of. If you're not Japanese, then, there is literally no point in getting them. Buy something that will last for years.