Quoting Ben Finney <bignose@debian.org>:
If I understand correctly, the justification of putting a file there must include that it is overwhelmingly more likely to save *storage space* overall (by reducing the space in a corresponding number of ‘/usr/share/doc/…/copyright’ files), especially on machines that have low disk space in ‘/usr/share/’.
Yes. This comes from times, when disk space was expensive. Times have changed for most systems, and those who work an small embedded systems (I do!) have brutal methods to solve the problem, mainly `rm -rf /usr/share/common-licenses/`.