Declarer claims the rest of the tricks. Why are all hands revealed?

When declarer claims the remaining tricks, you, as a defender can see all the remaining cards to help you decide whether you wish to accept or decline the claim. However, the declarer cannot see your cards nor those of your partner, nor can they if your side declines the claim and play continues for illustrative purposes.
See our Home Page > Information > Bridge > Advice from TD > Claiming