mistborn – When does using a Shield help?

Going over the rule about Conflict regarding Action/Defense Dice, it’s not clear when Shields (which grant Defense Dice against physical attacks) are useful.

Do you always roll Shield Dice when you can make a Defense Pool against a physical attack? Do you only get the Dice when you use your Action to Defend and/or when you use your Action for something that doesn’t cause you to gain Action Dice nor Defense Dice (but are physically attacked during that round)?