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.