[Geoff St. Germain]

Well, the reason to apply them (in reference to a subwoofer) is like you said, to have the driver work in only a certain band of frequencies. In the case of a subwoofer there would be a low-pass so that only frequencies below what the mains are playing are sent to the subwoofer and there could be a high-pass to prevent very low frequencies that have the potential to damage a subwoofer. For instance, for vented subwoofers frequencies below the tuning frequency result in very large excursion demands which could cause the driver to attempt to move further than it can mechanically resulting in what is called bottoming which can result in mechanical damage to the driver.