下面这段程序是什么意思，请指教！

<script language=JavaScript>
<!--
//

var version = "other"
browserName = navigator.appName;
browserVer = parseInt(navigator.appVersion);

if (browserName == "Netscape" && browserVer >= 3) version = "n3";
else if (browserName == "Netscape" && browserVer < 3) version = "n2";
else if (browserName == "Microsoft Internet Explorer" && browserVer >= 4) version = "e4";
else if (browserName == "Microsoft Internet Explorer" && browserVer < 4) version = "e3";


function marquee1()
{
if (version == "e4")
{
document.write("<marquee style='BOTTOM: 0px; FONT-WEIGHT: 100px; HEIGHT:110px; TEXT-ALIGN: left; TOP: 0px' id='news' scrollamount='1' scrolldelay='10' behavior='loop' direction='up' border='0' onmouseover='this.stop()' onmouseout='this.start()'>")
}
}

function marquee2()
{
if (version == "e4")
{
document.write("</marquee>")
}
}

function MM_swapImgRestore() { //v3.0
var i,x,a=document.MM_sr; for(i=0;a&&i<a.length&&(x=a[i])&&x.oSrc;i++) x.src=x.oSrc;
}

function MM_swapImage() { //v3.0
var i,j=0,x,a=MM_swapImage.arguments; document.MM_sr=new Array; for(i=0;i<(a.length-2);i+=3)
if ((x=MM_findObj(a[i]))!=null){document.MM_sr[j++]=x; if(!x.oSrc) x.oSrc=x.src; x.src=a[i+2];}
}
//-->
</script>
这段程序有什么用处，我看到它经常出现在某些程序的开头？