The kidneys are involved in maintaining the fluid and electrolyte balance of the body. They achieve this through their functional units called the nephrons, which are microscopic parts that make up the mass of the kidneys. In simple terms, the nephrons can decide how much or how less of water to reabsorb depending on the need of the body.
Technically, causing an increased reabsorption of water from the kidney nephrons will also cause the blood volume to increase. But the range of this is very forgiving so one can also argue that blood volume is maintained. One reason why reabsorption might be necessary is because the blood vessels in the body might have dilated for some reason and we need the extra volume just to fill up the excess space created by the dilation.