Sometimes your home bank has atrociously high commissions or fees on any international ATM withdrawal, or perhaps if it is not at an ATM that's in their preferred network (which may be a pain to go out and seek when you are in a foreign country). That, and also the possibility that ATMs you encounter may be low on cash or not working - all of it makes it a reasonable measure to get some cash in advance of your trip to get through the first few days.