Answer:
By agreeing to give birth to Jesus (God incarnate) Mary enabled the plan of salvation to be put into effect. Without Mary's "fiat" (Let it be done) God would not have been able to be born of a human being. \
There are some who think that it is possible Mary might not have been the first woman to be asked, but that she was the first to agree; this is not, however, the official teaching of any Christian denomination.