Alex, for many food products, storing them in temperatures in the 40's in a root cellar is tantamount to free refrigeration, even better actually because the storage can be drier than what you get in a fridge. And once the root cellar is built, that "refrigeration" is free going forward. Think of all the money and resources that are consumed on this planet by refrigeration.
If one lives anywhere with a short growing season, then you really want a cool temperature place to store lots of the things you raised in the garden during that short growing season (e.g., onions, carrots, potatoes, garlic, etc.) You want those things to last into the growing season of the following year, and a root cellar can make that happen. It is also a fabulous place to store foods canned from that short growing season--they will last for many years with no spoilage. So all those strawberry, blueberry, and raspberry preserves we put up this year will be with us for as long as we can resist devouring them or giving them away as gifts.
Also, if one is a baker, milled flour lasts very nicely in cool storage, and whole grains are likely to last for one's lifetime.
Did I mention: I really like root cellars and find them extraordinarily valuable! It's very unfortunate for all of us that they have "gone out of style."