They are not NECCESSARY but they are NICE to have under certain circumstances. I have two front wheels, one with a slick the other with a studded Nokian. When it is snowy (packed snow from cars) or icy, I put the Nokian wheel on and notice that I ride with much more confidence and speed than my commuting friends do. They create a lot of rolling resistance, though, so I don't like them when the pavement is clear.