Question

for longer operational life why dry cells should be used intermittently

Answer 1

Dry cells must be used intermittently as they recuperate themselves in the rest period by the action of a depolariser. Also the potential remains constant during its life as the overall action does not involve any ion in solution whose concentration can change during its life time.

Answer 2

In the primary/dry cells the potential remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its life time.