Files with .NOMEDIA file extension can have several uses. Below is a list of the most important of them:
Exclude certain files from indexes generated by folder scanners or other applications to boost system performance.
Hide multimedia files used by other applications. There is little reason to display application icons or audio file thumbnails in image gallery app view.
Hide multimedia files from unprivileged users who should not access such files. This ensures that only the person who knows the location of such files is able to access them.
Changing existing file to .NOMEDIA file in order to hide its contents or make accessing said contents harder.