Thematic buff card for
is really the main purpose.
Life is about adaptation. This represents creatures adapting to a dark and dangerous environment by coming nocturnal and more vicious.
Mechanics wise, it gives a way to extend nightfall and eclipse buff to creatures of other elements. The counterbalancing HP loss is to prevent abuse with sparks or other 0 HP creatures.
The ATK bonus is pretty substantial, so it should be useful in its own right...
Where would I use its?... giant frogs, forest scorpions, cockatrice. After its rolling its like adding +1
to get +3 ATK (unupped) or +1 :darkenss for +5 ATK (upped). That seems worth it to me... or would you really say a 10|2 Giant Frog is not worth it?.