The sand gets wet and there is always the chance for mold or germs etc to grow in it, grass and other bits get in it from time to time and well it started one year when dealing with the ice and being out of the salt stuff and ran out to the sandbox and managed to scrape off enough to put on the sidewalk and then the pattern was established. We have one of those green lidded Rubbermaid bins that sits on our front porch filled with the sand and we mix in a couple bags of the sand/salt mixture at the one end for stubborn days or a couple spots that are worse from where eaves drip.
Just like anything we clean and sanitize it just made sense to put in new sand each year. My box is only 4x4 so not a big issue.