Why does this CSS example use "height: 1%" with "overflow: auto"?

I am reading a HTML and CSS book. It has a sample code of two-column layout.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
    <style>
        #main {height: 1%; overflow: auto;}
        #main, #header, #footer {width: 768px; margin: auto;}
        #bodycopy { float: right; width: 598px; }
        #sidebar {margin-right: 608px; }
        #footer {clear: both; }
    </style>
</head>
<body>
    <div id="header" style='background-color: #AAAAAA'>This is the header.</div>
    <div id="main" style='background-color: #EEEEEE'>
        <div id="bodycopy" style='background-color: #BBBBBB'>
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
            This is the principal content.<br />
        </div>
        <div id="sidebar" style='background-color: #CCCCCC'>
            This is the sidebar.
        </div>
    </div>
    <div id="footer" style='background-color: #DDDDDD'>This is the footer.</div>
</body>
</html>

The author mentions that the use of overflow auto and 1% height will make the main area expand to encompass the computed height of content. I try to remove the 1% height and tried in different browsers but they don't show a difference. I am quite confused of its use. Any idea?

Solution

This is related to the hasLayout bug which manifests in IE6/7. The height: 1% is one of the common fixes. The other ones being zoom: 1; (which doesn't validate in CSS) and overflow: auto;.

Thus, if you remove both height: 1%; and overflow: auto; then IE6/7 may suffer from the hasLayout bug. Whether you would see difference or not depends on the future development on the template.