What was the meaning of Purim to the Old Testament Jews?

It is unclear what the question is actually asking.

1) The Book of Esther, upon which Purim is based, came at the end of the Old Testament, so if this is referring exclusively to the time period of the Old Testament, Purim would not have meant anything because it did not exist.

(It would be like asking what did the Treaty of Paris of 1783 mean to the American Colonists during their early history.)

2) Jews today still use exclusively the Old Testament and Jews claim that their religion has not changed since those days. To Jews today, Purim signifies how God intervenes and protects the Jewish people even without performing a Deus Ex Machina like he did in Egypt.