Why do 0 dB attenuators exist?
A 0dB attenuator is an attenuator that is electrically and mechanically just like other attenuators in that product line.  

You can use them in place of another attenuator when doing tests.  It acts just like the other attenuators (in terms of insertion loss and frequency range) and fits in exactly the same space mechanically.

A 0dB attenuator meets all the specifications of the others in the same product line, so swapping it in keeps all other performance characteristics of your setup the same.

Might not matter if you are tuning a CB radio, might be critical in high performance, cutting edge stuff.

The point of using one in the referred paper seems to be to reduce the amount of heat seeping into a very sensitive piece of equipment.  Heat causes noise in the extremely low temperature circuits the paper describes.

The 0dB attenuator seems to be used because it passes the RF signal (almost) as well as the cable, but doesn't pass heat as easily as the cable would.