Like Pnina, I don't use this option at all because if I want to do another render it's because I changed something significant in the scene and so the previous calculation is no longer applicable. If I changed something so small that the previous calculation still applies, then a simple preview quality render of that small area will be sufficient and so the "reuse" option is irrelevant.

So, in practice, for still images, I find the "reuse" option either irrelevant (because the change was too small and so a full render isn't necessary) or dangerous (because it could provide a completely different result than expected). So, I don't use it.