iPhoto Library

1. They’re structured for and used by different applications.

2. If you don’t use one of those applications, you can delete the library for it, but doing so won’t free up nearly as much space as it seems; the media files are hardlinked.

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Actually there's no difference Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

So you can delete the old iPhoto Library if you want, as always be sure to have a backup.

Inside the packages.

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