When it’s relatively warm out you can usually get away with a less than perfect sleeping bag. You might end up being a little cold, but it won’t be the end of the world. You might even get a full night of sleep in.
But when conditions get colder, like in to the sub zero range, you need gear that will perform, and perform well in those conditions. A sub zero sleeping bag, in particular, is a critical piece of gear to have in those kinds of conditions.
One of the more important things to look for when getting a sub zero sleeping bag is the cut of the sleeping bag. You definitely do not want a rectangular shaped sleeping bag, but even a wider mummy cut would likely not cut it for those cold temperatures. You will want a very narrow sleeping bag, which is sometimes called an alpine mummy cut.