What is the purpose of manganese dioxide in a Duracell battery?

Manganese dioxide is used in all brands of dry cell batteries (both the old acidic electrolyte formula and the modern alkaline electrolyte formula) as a catalyst to speed the conversion of microscopic hydrogen gas bubbles that form on the zinc electrode during use back into water. If allowed to accumulate these hydrogen bubbles raise the internal resistance and cause a battery to appear to go dead long before it has died.

So called "rechargeable alkaline batteries" are not actually rechargeable, their "charger" just applies a small reverse current which further speeds conversion of the microscopic hydrogen gas bubbles back into water. The correct term for this process is rejuvenation, as all it does is reduce internal resistance of the battery.