If a PC is running normally, if you press the power button just briefly the computer will go into its shutdown sequence--it will close and save all files and 'clean up' before it powers down. If you old the button down for five seconds, it forces the computer to power down immediately--like pulling the plug.
That COULD cause problems, but it's not likely these days, especially if you close all your files first. Windows looks for this damage and it can repair itself when it starts again. Though, of course, if you did a lot of work on a spreadsheet or text document you could lose some of it.
But if the computer is locked up or unresponsive, it may be your only option.