They are used a lot for generating normal vectors on the fly, since you can take in a whole triangle, you just do a cross product of two of the edges and you're

They are used a lot for generating normal vectors on the fly, since you can take in a whole triangle, you just do a cross product of two of the edges and you're done. From my experience, they are fast enough as long as you don't emit more than 2x primitives that you take in. They are also useful for culling primitives. They are not suitable for tessellation, which funnily enough is better done by tessellation shaders. Tessellation shaders are actually pretty flexible, and can do most of the things that you might have thought geometry shaders would be suitable for.