I've recently bought an external hard drive enclosure and it had arrived with an USB cable, with one USB connector on one side and two on the other. Why do some USB cables have two connectors while others do not?
It's because of power consumption. There is a limit of how much power a single USB port can provide. It is measured in "unit loads" which are 100 mA for USB up to 2.0 and 150 mA for USB 3.0. A single port can provide at most 5 unit loads for USB 2.0 and 6 for USB 3.0, if there are no other devices connected. Some ports may provide more, but they aren't standard compliant and manufacturers can't rely on them. With two cables, device may use at least two unit loads, so it will have enough power to work.