When to use the throttle module?

The throttle is useful to sites that have limited resources and usually are not that busy. It is actually less useful to consistently high-traffic sites, as by necessity they will already have the required infrastructure to survive high loads.

The throttle works well for a website that's on a shared host with limited CPU and bandwidth, that usually only gets a handful of visitors, and then is suddenly Slashdotted. Of course, it will only help in that case if it has been enabled and properly configured.