Debouncing IC input resistor?
The IC has an internal pullup resistor. Like most resistors in ICs it is loosely specified (a 3:1 range) and (because the manufacturer wants to maximize the market) rather on the high side. 



So the minimum switch current with a 5V supply is 50uA. Some switches would be more reliable with a higher current (often specified on the switch datasheet as minimum current) and if that is so you can add an external pullup resistor, so the effective pullup will be the parallel combination of the internal and external resistors.