Throttling Paradox

Who would have thought that you might need to throttle an application server in order to maintain a performance-level contract? I've seen the option inside IIS but couldn't for the life of me think why anyone would want to turn it on. However, in the context of an application that's been through some sort of capacity planning process (so we have a rough idea of how many requests/responses the application can process), and where all requests need their responses within some limited duration (think high-volume/low-latency trading platforms), you can apply throttling to help enforce the performance level. Without throttling, additional requests might be satisfied only to the detriment of other requests already executing on the application server. Ideally, it should be used in conjunction with load-balancing across a cluster of servers, so that throttled requests aren't queued, but rather dispatched to another less-busy machine.