Why do we pass null to XMLHttpRequest.send?

Why is send so often called as

xhr.send(null)

instead of

xhr.send()

?

W3, MDN, and MSDN all state that it's optional. Furthermore, the ActiveX control doesn't seem to need the argument:

hr=pIXMLHTTPRequest.CreateInstance("Msxml2.XMLHTTP.6.0");
SUCCEEDED(hr) ? 0 : throw hr;

hr=pIXMLHTTPRequest->open("GET", "http://localhost/books.xml ", false);
SUCCEEDED(hr) ? 0 : throw hr;

hr=pIXMLHTTPRequest->send(); // <-- this line
SUCCEEDED(hr) ? 0 : throw hr;

The practice of send(null) goes back at least as far as 2005 in Google Maps, but being minified, there's no explanation:

Y.asynchronousTransform = function (qc, vb, kc, Nc, Ba) {
    if (m.type == 3) return;
    var cc = Y.getCached(kc);
    if (cc) {
        cc.transformToHTML(qc, vb);
        if (Nc) Nc();
        return
    }
    var yc = qa.create(Ba);
    var sa = Xd.create();
    nd('<a href="' + kc.xmlEscape() + '">' + kc.xmlEscape() + "</a>", 0);
    sa.open("GET", kc, true);
    sa.onreadystatechange = function () {
        if (sa.readyState == 4) {
            if (yc.isValid()) {
                try {
                    var Db = sa.responseXML;
                    var cc = Y.create(Db);
                    Y.cache(kc, cc);
                    cc.transformToHTML(qc, vb);
                    if (Nc) Nc()
                } catch (b) {}