The reason is very simple, it makes your connection lesser bulky!
You will not bumper the cable / the hard drive accidentally, which may make the connection interrupted.
You may connect the Right Angle USB connector to your hard drive or the PC / laptop side, normally the latter.
This is a super speed USB 3.0 cable, makes sure your connection is at top speed.
You may learn more at:
If you would like a dual angled, here we have our UF005607
If you would like a USB 3.1 cable, here we have our UF004848