The cats kidneys process & filter water from the body, then recycle most of it back into the body, what is left is concentrated with the toxins for excretion so the urine in a normal healthy cat actually has little water content.
A cat that is chronically dehydrated will drink more to try to replace what they are losing through electrolyte imbalance & excess urination, the volume of urine output doesn't allow the body to catch up even though the cat is drinking high volumes.
Dehydration is not only the loss of water from the body but also the loss of electrolytes - body salts such as potassium & sodium they also get flushed out with the waste.
A cat with compromised kidneys is more likely to be dehydrated and is unable to concentrate the urine sufficiently so they need to drink & uriniate more to flush out the same amount of toxins that would normally be excreted with less urine.
Tus feels better directly after having fluids because it relieves the immediate symptoms of dehydration, flushes toxins faster, and generally makes her feel better. The IV & SubQ fluids is a delicate balance of not only sterile water but several electrolytes as well, so when Tus has fluid therapy not only is the water balance restored but so is the electrolyte balance.
A dehydrated cat will also refuse to eat in part because of the "stomach ache feeling" that happens when low on fluids but also in part because they just feel cr@ppy.
Avoid biting when a simple growl will do
The Spirit Lives As Long As Someone Who Lives Remembers You - Navaho Saying
Vindication ~ For all those pets who became sick or lost their lives from tainted pet food