Why is the adrenaline in the deck?
What kimham8a said above. It synergizes very well with the cards inside the deck.
FFQ is a 3 hitter.
Maxwell is a 3 hitter.
Uli is a 3 hitter.
Crusader is a 3 hitter, and when equipped with a pulvy, is an 8 hitter.
Lava destroyer is a 7 hitter when ungrown.
It can be used on an oty once it consumed something for extra damage.
Can be copied with twin universe.
It can be cast on produced fireflies for 5 extra damage and more
produced. (This makes me think that an unupped FFQ can also work. Unupped fireflies will have 3 attack and do an extra 9 damage with adrenaline, but this will ultimately be up to the user to decide)
And finally, it has synergy with SoSe, able to turn weak or small critters into heavy hitters.
For reference, 3, 7, and 8 are the three attacks that benefit the most from adrenaline, gaining a +9, +8, or +9 boost to damage.