A chain of why
s:
$ dpkg -S /usr/share/dict/words
diversion by dictionaries-common from: /usr/share/dict/words
diversion by dictionaries-common to: /usr/share/dict/words.pre-dictionaries-common
wamerican, dictionaries-common: /usr/share/dict/words
$ aptitude why dictionaries-common
i hunspell-en-us Depends dictionaries-common (>= 0.10)
$ aptitude why hunspell-en-us
i libenchant1c2a Depends aspell-en | myspell-dictionary | aspell-dictionary | ispell-dictionary | hunspell-dictionary
i hunspell-en-us Provides hunspell-dictionary
$ aptitude why libenchant1c2a
i libwebkitgtk-3.0-0 Depends libenchant1c2a (>= 1.6.0)
$ aptitude why libwebkitgtk-3.0-0
i unity-control-center Depends libwebkitgtk-3.0-0 (>= 1.3.10)
Of course, this doesn't really answer why it needs this list of words. Just that a chain of dependencies caused it to be there. The other chain of why
s might be a better answer, but:
$ aptitude why wamerican
i cracklib-runtime Recommends wamerican | wordlist
$ aptitude why cracklib-runtime
i libcrack2 Recommends cracklib-runtime
$ aptitude why libcrack2
i libpwquality1 Depends libcrack2 (>= 2.8.12)
$ aptitude why libpwquality1
i unity-control-center Depends libpwquality1 (>= 1.1.0)
More plausible: the list of words is used to mark passwords present in a common dictionary as poor quality.