We always had outdoor cats around when I was a kid. They never used our sandbox. Not that Daisy is wrong, but it's just not always the case.
If the boxes were to be used for propagating cuttings, they would have to be in the shade.
They may have been used to store marginally winter hardy plants in for the winter, that would be replanted in spring. Sand is also a good way to store root crops (carrots, beets, horseradish, etc.) over the winter.