Why Winter Tires?

Newer generations of winter tires are designed with a more flexible rubber compound that is less prone to stiffen in colder temperatures.
Their tread is also designed to provide improved traction on wet, slushy, snowy, icy and dry cold surfaces. (Rubber Association of Canada 2007a).

A summary of tire types.
Here is a simple rating system for tire types commonly used in Canada:

• Winter tires: Best in winter, fair in spring and fall (but tread wear
  increases), poor in summer
• Summer tires: Best for summer, fair in spring and fall,
  poor in winter
• All-season tires: Fair in spring, summer and fall, less satisfactory in
  colder conditions (7°C or colder) (Russell 2011)

Recognizing winter tires: A three-peaked mountain snowflake symbol is placed on all winter tires that achieve performance-based
standards. This symbol is used and recognized globally.

Winter Tires Vs. All-Season Tires

What is the difference between winter tires and all-season tires? On dry pavement, with temperatures just below freezing, the stopping distance for vehicles equipped with all-season tires is 30% longer than for vehicles equipped with winter tires.

Winter tires also have better traction on a snowy surface at temperatures of -40°C than all-season tires have at +4°C (Mahler 2008a).

Even on dry pavement, the overall performance of an all-season tire declines as the temperature becomes colder (Fountain Tire 2011).