Block is the only starting ability that lets you mitigate damage. Because most of the time damage happens either during a card chain or after your turn ends, Block will almost always be used as a reaction.
Hide allows you to evade baddies; Block allows you to mitigate damage from baddies and other sources.
For example, on the last move of her turn, Amy uncovers a baddie. Since she's out of action cards, she ends her turn and draws up to her hand size. Fortunately, she draws not one but two block action cards!
The baddie attacks and deals 1 damage. Amy plays her Block as a reaction but misses her roll. Drat! But wait - all is not lost. She plays her second Block, and this time she is successful! No damage taken! However, since she's already drawn for this turn, she'll be starting her next turn two cards down.
She was allowed to play a second Block because her first one failed. If she had two baddies on her, she could play a successful block against each of them. However, if a single card would do 2 damage, she could only Block to negate one of that damage.
Block only mitigates damage; if an effect tells you to lose a card, Block can't help. Remember, Attacks deal Damage, and Damage is resolved by Losing cards.
(Attacks->Damage->Losing cards)