Answer:
Transpiration is the process of evaporation of water from the leaves and stems to the atmosphere. The stomata close when the guard cells shrink, which in turn ceases the transpiration. Opening of stomata allows the process of transpiration, so the root cells do not die.
But if it can't close:
The downside to wide open stomata is increased water loss (transpiration). So stomata of cool-season turf grasses have decisions to make. The plant needs to capture and store energy via photosynthesis, however, excessive water loss through transpiration can result in plant injury or death.
It's a bad thing.