Weird USB cable (usage?)
What is this weird USB cable
The cable is called a USB Y-Cable. It is designed for devices which require more power than is available from a single USB port.
What are the 2 different kinds of Y-Cable?
One male connector, two female connectors
There are two types of the one male two female cables. One is a basic charging cable, splitting one port's power across two devices. The other is a specialised cable that somehow splits the data lines - and it's only used in very specific applications.
Two male connectors, one male or one female connector
What happens is you have your USB cable, one side for the host and one for the device, but with an extra connector attached. This extra connector does not have any data lines (D-, D+); it only has the power lines (GND, VBUS). It's attached in parallel to the existing cable. In other words, VBUS is connected to VBUS and GND is connected to GND.
Source Can I safely connect the power-only-end of a USB-Y cable to an other power source? answer by Bob)
So what is this weird Y-Cable?
The extra power plug is sometimes a double-sided plug, with both male and female.
It's a pass-through so that extra port used isn't totally wasted.
The female connector on the back of the plug allows a low-current device to use the second port.
Power requirements
Some devices, such as high-speed external disk drives, require more than 500 mA of current[89] and therefore may have power issues if powered from just one USB 2.0 port: erratic function, failure to function, or overloading/damaging the port.
Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power and data, the other for power only) to plug into a computer. With such a cable, a device can draw power from two USB ports simultaneously. However, USB compliance specification states that "use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral", meaning that "if a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered."
Source USB
The cable is meant for USB powered devices that require more current than 1 USB connector can provide. I have that cable for my CD Writer.
The single connector at the end is connected to the device and the other 2 connectors are connected to the computer. I believe that the data is only provided at one of the connectors, but power is needed from both.