A caching proxy server will generally treat each URL as a distinct unique file based on the path and query parameters. However, the query parameters can, in the majority of cases, appear in a different order and produce the same output. For example, a dynamically generated image over these two URLs would end up with the exact same file image.jpg?width=300&height=200 and image.jpg?height=200&width=300 but would end up cached twice because the URL is different.
What the Query String Sort feature does is automatically rearrange the query strings in the background when selecting the cache key. This means the cached file can be found regardless of the order of the query parameters and the two URLs will be saved and returned as a single file.