At it's core, FastCache really is a simple caching DNS proxy server - it sits between programs on your computer accessing the Internet and the DNS servers that map domain names to IP addresses.
Normally a DNS query happens very quickly, in the neighborhood of around a couple hundred milliseconds - that's roughly a tenth or twentieth of a second. This means that the first time you go to a certain domain, it takes the normal amount of time to look up the domain name, but each subsequent time you return should be instant (as long as it remains in the cache). But what good is saving a FRACTION of a second!?!?
In and of itself, not of much value at all, but the beauty of DNS is that you don't normally just make a couple of requests a day - with normal browsing you can literally make hundreds of requests, and that starts to add up.
The biggest reason this will always buy you a time savings is because many domains are configured with timeout periods which are very small, some as little as a couple seconds, some a few minutes to an hour, with the rest normally in the one day range. Since FastCache can override the normal timeout period, you can help to eliminate duplicate requests.