For instance if you run the import post module for a phpBB import for the first time, it will import all the post from the phpBB board and place them in the vBulletin database.
All the posts will have a importpostid which is the original post id of the post in the phpBB board. Any original post from the vBulletin board before the import will have no importpostid, as they are original.
So when the module is run for a second time (another practice run, you change some setting, updated a parser, got more posts, etc) it will delete all posts from the vBulletin board that have an importpostid.
This is useful for getting timings or re-running modules that had issues that you have changed or updated.
If you have associated or merged users, the original vBulletin users will now have a importuserid, re-running the import user module will delete these users, if you don't have any associated users this isn't an issue.