From the sample code below which I got here, I don't understand why the value of registers are move to specific part in stack when byte-spilling is performed.

        pushq        %rbp
        movq        %rsp, %rbp
        subq        $96, %rsp
        leaq        L__unnamed_cfstring_23(%rip), %rax
        leaq        L__unnamed_cfstring_26(%rip), %rcx
        movl        $42, %edx
        leaq        l_objc_msgSend_fixup_alloc(%rip), %r8
        movl        $0, -4(%rbp)
        movl        %edi, -8(%rbp)
        movq        %rsi, -16(%rbp)
        movq        %rax, -48(%rbp)         ## 8-byte Spill
        movq        %rcx, -56(%rbp)         ## 8-byte Spill
        movq        %r8, -64(%rbp)          ## 8-byte Spill
        movl        %edx, -68(%rbp)         ## 4-byte Spill
        callq        _objc_autoreleasePoolPush
        movq        L_OBJC_CLASSLIST_REFERENCES_$_(%rip), %rcx
        movq        %rcx, %rdi
        movq        -64(%rbp), %rsi         ## 8-byte Reload
        movq        %rax, -80(%rbp)         ## 8-byte Spill
        callq        *l_objc_msgSend_fixup_alloc(%rip)
        movq        L_OBJC_SELECTOR_REFERENCES_27(%rip), %rsi
        movq        %rax, %rdi
        movq        -56(%rbp), %rdx         ## 8-byte Reload
        movl        -68(%rbp), %ecx         ## 4-byte Reload

And also, I don't know what is the purpose of byte-spilling since the program logic can still be achieved if the function is the one saving the value of the registers it will be used inside it.

I really have no idea why is this happening. Please help me understand this.