I can envision a few uses:
1. Port saver. For some jack that gets plugged/unplugged constantly, leave the extender on the device so it wears out, and once it does, just replace the extender.
2. Connecting things without cables. Lots of things like filters, amplifiers and attenuators, exist as little metal boxes for inline use, with a male BNC on one side and a female on the other. To connect those directly onto the input of a piece of test gear, you might need a few cm of extension to get around the case or knobs or something.