I have seen this explained somewhere before but can't find a reference now. An example usage from docker getting started guide for Ubuntu:
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
I even recall very vaguely what it is supposed to do. IIRC correctly, it is supposed to somehow use the stdin passed through the pipe. But why is this necessary in this case?
The closest I came to finding anything about this was in this article: http://www.catonmat.net/blog/bash-one-liners-explained-part-three/. Maybe, it's related to closing a stream, like in the example where you open a non-standard stream and then close it?
exec 3<file exec 3>&-