A dam can have floodgates to control the flow of water (and therefore the water level in the reservoir to a degree), not just a weir with a fixed elevation where water flows over the crest, so I suspect that you might confuse a dam with a weir. I read that a professor said this particular dam had 26 gates. If you expect a flood, you’d want to open the gates beforehand, as to make room in the reservoir, and to reduce the peak flow that an uncontrolled flood would give, for example.
Because they want to flood a lot quickly? They don’t just want to spill the overflow. Are you confusing dam and sluice?
A dam can have floodgates to control the flow of water (and therefore the water level in the reservoir to a degree), not just a weir with a fixed elevation where water flows over the crest, so I suspect that you might confuse a dam with a weir. I read that a professor said this particular dam had 26 gates. If you expect a flood, you’d want to open the gates beforehand, as to make room in the reservoir, and to reduce the peak flow that an uncontrolled flood would give, for example.