What role does the dummy play in bridge whist?

In bridge whist, the dummy is the partner of the dealer. After the opening lead is made by the player to the dealer's left, the dummy places their entire hand faceup on the table, grouped by suit.12

The dealer then plays both their own hand and the dummy's hand, making plays from each in the proper turn. This exposure of the dummy hand was a significant innovation over traditional whist, as it provided clarity in many situations where a whist player would otherwise have to guess, thereby creating new opportunities for analytical and skillful play.123