Sepples with structs

Every time I see that posted I wonder how that'll work in practice. I'm interested to know if there is any real world example of that in from of a useful library. Filesystem, Graphics, Threads, Algorithms, or whatever is there.
And obviously not a C library,

Why the fuck would anyone using C++ revert to using shittier constructs voluntarily? It's not like you get any performance benefit out of it. In fact it would be slower because you'll end up re-implementing vtables when most compilers don't even use them.