I also read somewhere that a core file is created because with each new WordPress version, some functions become out of date and some files become redundant. But the thing is this update does not automatically delete those old core files.
According to some, these old core files can exposed your sites to hackers. So for security, it is better to have them deleted and besides, these old files are of no use, except taking up space.
One easy way to find these unnecessary old core files is with the help from a plugin.
1. Go to Old Core Files plugin over at: http://wordpress.org/plugins/old-core-files/
2. Install and activate the plugin.
3. At WordPress admin, scroll down to Tools and then click on Old Core Files.
4. Immediately you can see the report as shown below:
5. As you can see there are two categories: 'Existing' and 'All'.
6. You ONLY have to delete files which are listed in the Existing category.
7. Currently this plugin does not have the delete function.
8. You need to delete the core files manually via FileZilla.