There are people who say "Snow tires aren't needed!" They'll tell you "Advances in year round tires have obsoleted them!" They might even pull the "I have all wheel drive!"
These are the same people who told you heroin and AP Physics C were good life decisions.
Snow tires are absolutely vital to safe and effective driving during the winter months. All season tires typically are made of hard rubber compounds and have relatively small grooves, or fillets, in them to clear water. This is excellent for summer driving: the hard rubber means they will last for many miles before needing to be replaced and works together with the small fillets to decrease rolling resistance and increase fuel mileage.
Both of these traits are the absolute last thing a driver needs after it has snowed. The hard composition means the tires are more likely to spin instead of roll, and the small and narrow fillets quickly fill with snow and prove ineffective. Luckily, someone invented snow tires. Snow tires are nearly polar opposites of all season tires; their soft rubber compound and deep, chunky groove greatly improves their grip.
There are also those who claim that while snow tires are good, studded tires are better. This is a bald faced lie for any surface but sheer ice. Snow tires' ability to mold to the surface increases their contact patch and therefore the amount of force they can transfer from the car to the road. The studs on studded tires vastly decrease their contact patch, forcing the entire car to rely on several small metal spikes.