What's special about ceiling paint that makes it particularly suitable for ceilings?
Ceilings are typically painted with a flat sheen paint, as it's good at hiding (or at least not highlighting) imperfections, nor does it need to be particularly durable. Those two qualities tend to lead to a less expensive paint, and if you can save money where you can, why not. If you've already got some leftover (non-glossy) paint, there's no real reason not to use it. I wouldn't use gloss or semi-gloss on the ceiling, but eggshell should be OK.