Benefits of Kapost Cloud:
If you want to generate a tracking URL (i.e. the bit.ly URL Kapost uses to track downloads and views), a document must be published somewhere (it can't just be marked as published). Since Kapost may not integrate with all tools that you might want to publish content to, Kapost customers can just publish to the Kapost Cloud. Doing so will generate a published URL and a Tracking URL.
Republishing to the Kapost Cloud will replace the document without updating the URL (that is the desired behavior). Our recommendation is that you just publish/house all of your assets in the Kapost Cloud (as there is no file storage limit).
However, if you want to make changes to the content and republish it, all you should do is mark the content as published and then take the updated document and manually replace the original. This will keep the URLs intact.