Blizzard is one of the most powerful witchers' potions; it improves reflexes and reaction time, allowing a witcher to better parry and evade attacks. It slows down time, making Geralt's movement and everything else slower. It is most helpful against groups of enemies and monsters that move fast or teleport, such as wraiths and various types of vampires.
"Witchers usually drink the Blizzard potion immediately before combat. The potion is also favored in especially dangerous situations."