Why use a resistor for Pull-Up?
You have to use a resistor so the pushbutton can overpower it.  If you use a piece of wire, you get a short between Vcc and GND when you push the button - not so good for the button or the rest of the board.  Using a resistor holds the pin at a specific level until something 'stronger' (lower resistance) comes along and changes it (in this case, the button).