In temperatures below 7 degrees standard summer tyres begin to drop in performance levels due to the rubber stiffening because of the nature of compounds used in summer tyre production.
By contrast Winter tyres are designed to excel in temperatures below 7 degrees due to a high silica content within the compound. This ensures the tyre retains flexibility in low temperatures and will give a much better braking and traction performance on wet roads in cold conditions
Whilst winter tyres are not compulsary in the UK they are certainly beneficial in low tempertaures and in icy, snowy conditions
Please see a video below from Pirelli for some comparisons between summer and winter tyres