WakeMeOnLan is a simple tool which enables you to turn on sleeping networked computers by sending them a Wake-on-LAN packet.
When you first launch the program it'll scan your wired network (it can't be used with wifi), collecting the network name, MAC addresses and other details of everything it finds. Which makes WakeMeOnLan useful as a simple network scanner.
Later, though, if a network PC has been turned off or is in standby, you may be able to restart it by selecting the system in WakeMeOnLan and clicking the "Wake" button.
And the program even supports command line options to turn on a specific computer or IP address. Build this into a script or scheduled task, say, and you could turn on a network PC before you run a backup job which will save data to its drive.
Don't be surprised if this doesn't work immediately. Wake-on-LAN only delivers results if the remote computer supports it, and it's enabled (you'll probably have to configure a BIOS setting, and maybe a network card or power management option, too). If there are problems then browse your system documentation for details.