Checking if a variable exists

Why does this code check whether ‘oneHref’ is true or not in the beginning of the ‘if’ statement? I took it out and the code still preformed its function (to add an <img> to external links on page load.) So, if it still works why should it be there?

<script type="text/javascript">
			function externalLinks() {
				var allLinks = document.getElementsByTagName("a");
				var icon = document.createElement("img");
				icon.setAttribute("src", "external.png");
				icon.setAttribute("alt", "[external link]");
				for (var i = 0; i < allLinks.length; i++) {
					var oneHref = allLinks[i].getAttribute("href");

                                        // here, right before the '&&'
					if (oneHref && oneHref.indexOf("http") == 0) {
						var newImg = icon.cloneNode(true);
			window.onload = externalLinks;

(code taken directly from Designing with Web Standards, 3rd Edition. Jeffrey Zeldman)

If there are links in the page without an href it will throw an error without it, it’s just being safe.
e.g. calling indexOf on an undefined or null object will throw an error.

if (undefinedVariable.indexOf("http") == 0) {

In properly marked up code all <a> tags should have an href attribute, correct? Is it legal to have an anchor tag without an href? If so, for what purpose?

Also, what are the consequences of throwing an error? I edited the code so that it’d throw an error - and it did - but it still worked (the external link image was still placed after the links with an href attribute)