My RB Amber had kidney failure.

The sub Q fluids are to maintain hydration. In kidney failure, the kidneys over act and deplete the needed moisture in the body. The fluids help keep up the body's other systems by holding the fluid level stable. If you stop it completely, then the slide down hill is quicker and more difficult to reverse; and other systems are affected. You can test for the hydration level by pinching a bit of skin on the side. It should bounce right back to normal. If it holds for a second then slowly releases and drops, there is not enough fluid in the body.

Hope that helps!

Here is one site (of many) with info.
http://www.felinecrf.org/symptoms.htm