It's generally used to allow rich media ads (e.g. flash anims) to be embedded as well as images. Though this wouldn't solve the problem, a slightly better solution would be:

<NOLAYER>
 <IFRAME SRC="..." HEIGHT=60 WIDTH=468 MARGINWIDTH=0 HSPACE=0 VSPACE=0
+ FRAMEBORDER=0 SCROLLING="no" BORDERCOLOR="#000000">
  <A HREF="...">
   <IMG SRC="..." WIDTH="468" HEIGHT="60" ALT="Click Here." BORDER="1"
+ NATURALSIZEFLAG="0" ALIGN="TOP">
  </A>
 </IFRAME>
</NOLAYER>
<ILAYER ID="advert" VISIBILITY=hidden WIDTH=468 HEIGHT=60>
</ILAYER>
[download]

then at the bottom of the page:

<LAYER align=center SRC="..." visibility=hidden id=layer1 width=468 he
+ight=60 onload="moveToAbsolute(advert.pageX,advert.pageY); layer1.cli
+p.height=60;layer1.clip.width=468;visibility='show';">
</LAYER>
[download]

this causes netscape to load the ad after the page content, and then moves it to the position required.