A proxy for the remote content – Dump servlet. Let us see the typical case – you are linking to the image on the other site. Yes - it is so called hotlinking. You are eating the traffic from that site.
Dump servlet in this case lets you request the remote content only once – during the first call. After that it will be cached and served right from your host.
Another possible use case for this servlet is a cache for the remote CGI scripts. Actually servlet supports both – client side and server side caches.