throt-tle (throt'`l), n. the valve in an internal combustion engine that regulates the amount of gaseous fluid entering the cylinders. v.t. to reduce the flow of a gaseous fluid by means of a throttle, hence; to slow (down) by this or similar means

1. You want or need to leave some of your bandwidth available for other things.
2. Your ISP might penalize you (by applying a severe throttle of their own) for using all available bandwidth too much of the time.
3. Your stupid modem might otherwise spend all its time sending and not be able to receive hardly any data at all at the same time:

Notes on Throttle Settings

1. Setting an enabled throttle to a value higher than the full available transfer rate of your internet connection is essentially meaningless.
2. The bigger the throttle cycle value, the less often WinMX will apply the throttle.
3. Applying the throttle more often than necessary for good adherence to your specified rate of data flow just wastes your computer's resources.
4. Some modems won't receive worth a damn if they're also (always) sending; applying just enough of a throttle so that the modem is no longer sending all of the time should greatly improve the modem's ability to move data in both directions concurrently.
5. Nothing beats a trial like a failure. Your mileage may vary. So if you have questions about how this works, try various settings on your computer...