The built-in iMac monitor is very limited in the video modes it supports, and the ones it does support are "nonstandard". (It has a fixed horizontal scan rate which gets divided by larger or smaller numbers to produce the various vertical resolutions.) You could "in theory" drive it with another computer, but you'd have to custom-program appropriate video modes. Pretty sure that just 'plugging and going' via that 15 pin connector doesn't work on anything but an iMac motherboard. (Which has the appropriate video modes present in its firmware and recognizes that monitor when it's plugged in.)
What's more useful is being able to plug in a different monitor to your iMac's motherboard when the built-in one dies. ;^b