I'm not them but whenever I've used it it's been for arch specific features like adding a debug breakpoint, synchronization, using system registers, etc.

Never for performance. If I wanted to hand optimise code I'd be more likely to use SIMD intrinsics, play with C until the compiler does the right thing, or write the entire function in a separate asm file for better highlighting and easier handing of state at ABI boundary rather than mid-function like the carry flags mentioned above.