It isn't. Mutliple runways are primarly used to increase capacity, not for the odd case every 50 years when the only runway is blocked and there is an aircraft that has an emergency. Prague is situated in the center of Europe with tons of airports around, where aircraft can easily and safely divert, should the single runway be closed.
Worth mentioning is that even if you have 2 or 3 runways, if a runway becomes closed due to emergency (let's say an aircraft on fire), there would likely be no landings on the other runway(s) due to insufficient fire and rescue coverage.