code:
int
PyTuple_SetItem(register PyObject *op, register Py_ssize_t i, PyObject *newitem)
{
    register PyObject *olditem;
    register PyObject **p;
    if (!PyTuple_Check(op) || op->ob_refcnt != 1) {
        Py_XDECREF(newitem);
        PyErr_BadInternalCall();
        return -1;
    }
    if (i < 0 || i >= Py_SIZE(op)) {
        Py_XDECREF(newitem);
        PyErr_SetString(PyExc_IndexError,
                        "tuple assignment index out of range");
        return -1;
    }
    p = ((PyTupleObject *)op) -> ob_item + i;
    olditem = *p;
    *p = newitem;
    Py_XDECREF(olditem);
    return 0;
}

olditem is not useful.

> olditem is not useful

It is. Py_XDECREF() may have massive side effects (such as calling a __del__ method and executing arbitrary code). Therefore, you have to ensure that the tuple item is set to the new value *before* the old value is DECREF'ed. Otherwise, any code invoked by Py_XDECREF will see invalid tuple contents, and the interpreter may crash.