Today’s Question: I use Lightroom Classic CC. When editing I save the edits to an XMP sidecar file. My understanding is these edits are also recorded in the catalog, so why do I need an extra XMP file on my disk which also slows down Lightroom? Will other programs use the XMP file should I ever drop Lightroom and use another program? Are they used for another purpose?
Tim’s Quick Answer: If you save metadata for your raw captures to XMP sidecar files (or directly to the file for other image formats), standard metadata such as keywords and star ratings will be included. That metadata can be accessed by other software applications. In addition, adjustment settings from the Develop module are included, which could also be used by Adobe Camera Raw (via Photoshop and Bridge).
More Detail: By default, the various updates you apply to your photos within Lightroom Classic CC are only saved to the Lightroom catalog, not to the actual image files. If you manually save metadata to images (via the Metadata > Save Metadata to File command) or enable the option to have this information saved automatically (via the Catalog Settings dialog), Lightroom will save updates to the image files themselves in addition to being written to the catalog.
For standard image file formats (and Adobe DNG files), these metadata updates would be saved into the file itself. For proprietary raw captures the information would instead be written to an XMP sidecar file alongside the raw capture.
One of the reasons I prefer to enable the automatic saving of this metadata to the source image files is to provide a backup for the key information that would otherwise only be contained in the Lightroom catalog. Lightroom-specific features such as pick/reject flags and collections are not included in this capability, which makes those features dependent upon the Lightroom catalog. But I prefer to have this partial backup for my most important metadata updates.
The Develop settings that are saved along with the standard metadata are specific to Lightroom and Adobe Camera Raw. Therefore, while other applications can access the standard metadata values saved out to the image files, adjustments you’ve applied would require Lightroom or Camera Raw.
So if you switched to a non-Adobe software solution, the standard metadata saved to your image files would be accessible, while the Develop adjustment settings would not be available.