Low-level system code (particularly when you're doing things like servicing interrupts) needs to be written in assembler. The very early boot phase is even more annoying because you're starting out in one of the 16-bit modes and working your way to changing to x86-64 and virtual memory.

Outside of system code, the main use for assembly is trying to maximize performance on hot loops. Your optimized matrix-multiply routine, or media decoding kernel, could well be written in assembly. I've seen a few cases where people do things such as manual calling conventions in assembly as well.