When the gatherer completed taking the money, he would then put a leather covering on the remaining money in the treasury to ensure that when he came to take the next time he will take some from the new monies that were brought.

On the first gathering, he said this is from Eretz Yisroel for all the Jews. On the second, he said this is from the surrounding cities of Eretz Yisroel. On the third, he said this is from all the far away places.

Shikalim 4:1

The treasury money was first used for the 2 daily olahs, mussaf and their libations, the karban omer, the two loafs on shivous, the lechem hapanim and all public karbanos.

It was also used to pay the watchmen on shmita who watched grains that grew by themselves for the omer and the two loafs of shivuos. R’ Yosee says that anybody can do the watching for free. The Chachamim say no, if someone was doing it for free, he would automatically own the grains and these karbanos must come from the public.