Effect updates and description changes can be seen here.

Strength is a Bravery deck specific effect card. You can use it to give 2 or 2 extra attack to a monster.

It's advised to use this on a monster with high hp, attack, or a taunt if you want whatever monster is attacking to be gravely injured or killed.

Alternatively, it can be used to give monsters such as Memorial Statue, Papyrus Statue or Gaster a higher attack to protect them from Muffet's Pet, even though they cannot attack themselves. And it can be used on 0 ATK cards like Pyrope or Cactus to make them able to attack. And in the case of Cactus, Strength can activate its ability.

This card isn't used much, but, since it can give extra attack to a monster that is already placed for just 2g, and since it doesn't slow down the deck, it shouldn't be underestimated.