It's the basis of efficient relocations in position independent code, which is now very common.

PIC can be emitted on older x86 machines without RIP-relative addressing, but the code is larger and slower. As an example, consider -m32 gcc output for the C program

    int x;

    int getx() {
        return x;
    }

With no PIC:

    getx:
	movl	x, %eax
	ret

With PIC:

    getx:
	call    __x86.get_pc_thunk.cx
	addl    $_GLOBAL_OFFSET_TABLE_, %ecx
	movl    x@GOT(%ecx), %eax
	movl    (%eax), %eax
	ret

And with -m64, which emits PIC and uses x86-64s RIP-relative addressing:

    getx:
 	movl	x(%rip), %eax
	ret

Hopefully that makes the motivation clear.