Chem HW Question! Help quickly plz!?

In school, we did a lab where we had a beaker filled with ice and we put a test tube w/ water inside the ice filled beaker. We sprinkled salt on the ice in the beaker (not the test tube). The water inside the test tube froze after 15 minutes. I know we added salt to lower the freezing point. The thing i dont get is why we had to add it... was it so the water in the test tube would freeze faster or was it so the ice would stay cold? some other reason?? help!