It should work just like manually saving multiple times a rendering before it reaches the sample or time limit, but automated.
What should it be for? To test the time and sample needed for a picture, without having to stare at the image being rendered, or having to launch multiple renders of the same thing.
I imagine it like this: when starting the test rendering, set a comfortably high maximum time or samples value, and in the Options tab tick the Snapshots option on.
Next to it will appear some text boxes, those are for writing at what point each snapshot will be taken, time or samples being available for each slot (if no "h", "m", or "s" is written in the box, the system will assume the unit is Samples).
So let's say the maximum time is set to half an hour, and you set snapshots to be taken at 300, 400, 500, 600, 15m, 20m, 25m. You launch the rendering, and the system will progressively save a snapshot each time it reaches one of those values, and keeps on rendering until the deadline, instead of starting a rendering all over again. In the meanwhile you can leave the PC and come back later to see the results in your selected renderings folder.