Because if it ices over, your furnace will stop working on the day1 you most needed it.

Flushing it out from the outside with a bucket of hot water and a pump similar to this one 2 will melt the ice and open up the drain and you will get heat again.

1 Day because if your freezing weather lasts much longer than that, your homebuilder probably engineered a house that could handle the cold. Unfortunately my house was built by southern rednecks who’d never heard of insulation.

2 Not an endorsement of any brand or retailer. Just make sure it has a long enough hose. That’s what made it work better than some of the things I tried first.