1. Provide caching services, which are rarely seen on the client proxy.
This means that the actual server has less to do and doesn't have to re-send the same content every time the client connects.
2. Having multiple such proxies in different geographical locations allows clients close to the proxy to "feel" at least more responsive to cached content.
That's really what the CDN (Content Delivery Network) does, or the main part of it.
Just point the proxy to the image server before shutting down the master server for maintenance. This is not the only way to solve this problem, but it may help if you can't easily swap IP addresses to mirrors.
Of course, all of these benefits are due to the additional capabilities that the agent allows. Not just the fact that it "simulates" IP addresses and passes connections. This section is actually much more useful for server-side connections than from the client.