adjacency primitives

Hi all,
i have a stupid question: what adjacency primitives like lines_adjacency,triangle_adjacency used for? I tried to google it but didn;t find anything useful. I know how they work but i dont know their usage, shadow creating maybe?

Thanx in advance

They can theoretically be used for various things. The most notable is using geometry shaders to do some form of tessellation. But since GS performance doesn’t really make that viable, nobody does it.