Today’s Question: I’m still not understanding why XMP files are needed or how they are used [with Lightroom Classic]. Are sidecar XMP files only really important if you lose your catalog?
Tim’s Quick Answer: Saving metadata to XMP sidecar files from Lightroom Classic provides a backup for key metadata in case your catalog is ever lost, and also enables other software (such as Adobe Bridge) to see the updated metadata for those photos.
More Detail: In Lightroom Classic the information about your photos is stored in the catalog, and any updates you apply to your photos (such as updating metadata or applying adjustments) are by default only saved in the catalog and do not get reflected in the image files on your hard drive.
You can manually save metadata to selected images by choosing Metadata > Save Metadata to Files from the menu. You can also enable automatic saving of metadata to the source files by turning on the “Automatically write changes into XMP” checkbox on the Metadata tab of the Catalog Settings dialog. Just keep in mind that both of these options only cause standard metadata (such as star ratings, keywords, and more) to be saved to your source image files. Lightroom-specific features such as collections, virtual copies, pick/reject flags, and more are not included.
For raw captures the metadata is saved to an XMP “sidecar” file, while the metadata is saved directly into the source image file for other supported image formats.
The value of this metadata being saved to the source image files is twofold. First, it provides a backup of key metadata, which would be especially helpful if you lost your Lightroom Classic catalog and didn’t have a backup you could restore from.
Second, saving metadata to the source images enables the standard metadata updates you apply in Lightroom Classic to be visible in other software applications, such as Adobe Bridge. You need to be very careful not to actually update the metadata in those other applications, as that would create a metadata mismatch. However, it can be helpful to be able to at least browse some of your metadata updates outside of Lightroom Classic.