Compilers are great, but they don't always produce optimal code. You can quite often produce faster code if you're prepared to hand roll the assembly code, but it's rarely something most people will need to do. The time it takes to produce often outweighs any advantage you'd get, but having it as an option is going to be very important.

Just within the rust world, https://www.nickwilcox.com/blog/autovec2/ from a few weeks ago talks about the limitations with the compiler around vectorisation, demonstrating that with hand-rolled assembly they were able to cut the runtime in half, vs what the compiler was able to produce.

Outside of the rust world, you'll often see hand-rolled assembly in performance critical code across a spectrum of uses, e.g. OpenSSL uses it extensively for performance reasons, Go has it in a number of places in their libraries, with a pure go fall-back, and so on.