Portable HDD USB Cable?

I got my Portable HDD with USB 2.0 interface. But i'm curious about its cable, its a Y-Type, based on google search the Y-Type USB cable is use on portable hdd that requires additional power from usb port. Does it necessary to plug those 2 port prior to use the portable hdd? I try to plug only 1 and it works well. Do you think its necessary?? Thanks for the help..