It All Adds Up

The gifts Bnei Yisrael gave for the Mishkan were voluntary; people gave as much or as little as they wanted.
But another kind of Terumah offering, called the Machatzis HaShekel, was given by the people – no more and no less than a half-Shekel of silver. This coin was used to make the sockets for the wooden beams of the Mishkan to keep it strong and sturdy. This also kept all Bnei Yisrael strong and united together with their small but valuable contribution. Nobody could say, “I am better than you since I gave more!”
Once all the materials were collected it was now time to start building the Mishkan!