Which is a practical application of freezing-point depression?

Any place where the liquid you are using must remain a liquid at temperatures lower than its freezing point - the applications are almost endless.

As a simple example, if you were to try use the windscreen washers on your car on an extremely cold day, you might find the water has frozen and it will not work. If you had added a little anti-freeze, it would still be liquid and could also help melt any ice on the windscreen.

Think of the oceans, that are regularly below 0c. The salt (and other materials) in the water lower the freezing point. We would be in big trouble if that did not happen.

The effect can actually be very severe. I remember once having no dry ice or N2 on hand in the lab and having to concoct solutions of salt (calcium salts, I think) + ice from the freezer to get a liquid cold enough to use a shlenk line, eventually managing around -40C, which is pretty good going.