What is this cable used for?

[TheConfundedJerry]

I believe it is a USB 3.0 cable. It came with my U2515HX Dell Monitor, along with an HDMI cable.
What is it's intended purpose?

 

[USAFRet]

It's a USB 3.0 "B".

Shaped that way to prevent you from connecting a USB 2.0 or 1.1 cable
https://en.wikipedia.org/wiki/USB_3.0
A USB 3.0 Standard-B receptacle accepts either a USB 3.0 Standard-B plug or a USB 2.0 Standard-B plug. Backward compatibility applies to connecting a USB 2.0 Standard-B plug into a USB 3.0 Standard-B receptacle. However, it is not possible to plug a USB 3.0 Standard-B plug into a USB 2.0 Standard-B receptacle, due to a physically larger connector. The Standard-B is used at the device side.

 

[Glenwing]

I believe it is a USB 3.0 cable. It came with my U2515HX Dell Monitor, along with an HDMI cable.
What is it's intended purpose?

It's for making the USB ports on your monitor work.

Reading the manual will often answer these sorts of questions.