I'm not a game developer, but I write almost nothing but assembly code for a living (I'm a library writer). Generally this is for performance reasons, but I also do it to work around compiler bugs, or to use hardware features like condition flags that are actually easier to express in assembly than in C.
I'm usually writing complete functions in assembly, so I tend to write .s files that are assembled into object files and linked into an executable or library. People who just need to move a single loop into assembly often use inline assembly in their C source, which is supported by most compilers via some sort of intrinsic.
"Simple" examples are pretty rare, since if it was simple, the compiler would do a sufficiently good job and there would be no need for assembly.