While the primary purpose of Power over Ethernet is of course to provide electricity to your Ethernet-based IP Phones and wireless access points and other devices, the fact that all those cables go back to your highly-configurable, easy-to-manage and -monitor switches means that you actually have a very convenient way to cycle the power on those devices.
This can be particularly useful in those mysterious cases where a wireless access point's throughput begins to degrade significantly and just rebooting it occasionally seems to restore it to normalcy. Since the access points are often difficult to reach physically, you could log into the access point (typically via a Web browser) to tell it to reboot itself, but if it's locked up or you don't have the password, it's a lot easier to power it off from the switch, using whatever command your vendor recommends to disable the ports or power to the ports.
For IP phones, which are far more common, using the switch to reboot the phone can also be useful, but if your focus is on a single phone, it's probably still easier to reboot it from the configuration tool, such as CallManager. However, you might want all the phones to reboot at once, for instance, after you make some configuration changes, or as part of a monthly maintenance cycle. In this case, the configuration tool might be a bit cumbersome for bulk activity, and telling your switch to shut down all the user ports and then turn them back on could save a lot of time. Just keep in mind that when you reboot a switch, all the devices connected to it will come back up at roughly the same time, which can create a significant load on other systems, such as the DHCP server or CallManager.