What is the benefit of storing metadata in JPEG files? What are the implications if a JPEG doesn't contain any metadata?
Metadata as it called now in the digital photography age is information about the photo it is attached to.
In days of yesteryear information may have been written on the back of a printed photo, or on the slide mount or on an index card filed with the negatives/slides so that any information the photographer may feel is important to know and access is not lost to memory.
I was also important to professionals for ownership and copyright purposes.
Here are some interesting things to read.
Metadata; which are commonly EXIF, XMP, IPTC, and custom formats; contains information about the image like camera settings & other information like comments, dates, photo editing software information, ...
In the photography realm, camera settings (ISO, shutter speed, aperture, white balance, focal length, lens information, use of flash, other camera settings, ...) are commonly referenced. Most of this information is found in the EXIF metadata, but some interesting data is found in a custom metadata section. Photographers sometimes want to see what camera settings were used for images.
Some programs, like HDR and Panorama stitchers use EXIF metadata to make programmatic decisions on how to process the image.
You can have JPEG image files without meta data information. In fact, there are programs that can wipe metadata from a JPEG image files. There is no harm in having an image file without metadata.