keeping amps cool?

That's right. And some come with one that operates constantly. Some don't come with one at all. Hmmmm.

Cool is not to change the sound. It's o extend the operating life of the caps and perhaps the whole amp. Heat kills. Specifically I enjoy a Spectral DMA 180 and am happy with the sound-so would like to keep it a long time; without having to ship it across the US/Can border. Always some danger in shipping.

Unless it operates at the temperature it was designed to, it will definitely change the sound. Capacitors typically never get anywhere near their highest allowable temp. Those parts that are vulnerable to thermal breakdown are the ones on a heat sink. If your amp is functional after 20 years, it still needs to be rebuilt.

Why does it need rebuilding if it still seems to function fine?

That's just it. It 'seems' to function. Although it may sound okay, it has by then fallen probably far below spec. If you owned it since new, you probably didn't notice it's deterioration over the years. If it's a great amp worthy of a good overhaul, you're in for a treat. It will likely sound better than new due to current technology parts and maybe a mod or two by a respected tech or engineer.