hi
the pullup resistors used here for keping the controller pin in the high state when the switch is inactive (you are using a pull up resistor's one pin connected to uc and switch and the other to 5v the second pin of switch is grounded)the resistor helps to limit the current as only the voltage state is required for the controller
2/3VCC=high & 1/3VCC=low
so tahts all about using a resistor for pullup and the requirment of pull up