Dan Pfeiffer, writing in The Message Box:

The political logic of the Walz pick is less obvious than with Shapiro and Kelly, the other two finalists. This is especially true with Shapiro, a very popular governor from the critical state of Pennsylvania. Harris likely has no path to the White House without Pennsylvania.

And:

In picking Walz, Kamala Harris looked at something broader than winning one state. She sought a running mate to help her in all seven battleground states. Winning Pennsylvania alone is not enough to get to the White House. Presidential politics is more chess than checkers, and choosing Walz is evidence that Kamala Harris is looking at the whole board.

Lest we forget, the queen is the most powerful piece in chess.