Many 3D Applications these days store previous renders with all information and data. This lets you go back to old renders and review them or re-save them should you need to. This would be especially useful for comparing different settings (similar to how the "Show Original" slider works). Sometimes you need to compare AA settings or different colors and render times, but have to remember or log all the relevant settings.
Modo has a very nice implementation of this:
Roses are #FF0000
Violets are #0000FF
All my base are belong to you.