The authors of a free/libre work can change the license of their work any time, and they can even make it non-free again.
This does not affect copies that were made before that change (and any further copies from that "branch"), so I may still use/modify/distribute it according the original license.
Let’s assume the authors remove the license as well as all mentions that their work once was published under this license (and they forbid the Wayback Machine to publish older versions of their website).
I’m distributing their work (according to the terms of the original license). How can I prove that the work really was published under this license?
(I hope it will never be necessary to prove this, but better be safe than sorry.)