What are the function of manganese dioxide in a dry cell

It acts as a depolarizer in the dry cell

Explanation: Simple cells such as dry cells experience two major defects which are a disadvantage over the other cells. The two defects are polarization and local action. Local action is the "eating away" of the zinc plate which is the negative electrode in a simple cell. Polarization, on the other hand, is the formation of hydrogen gas on the positive plate of a simple cell. A depolarizer such as manganese dioxide is used to overcome polarization in a simple cell.