It reduces the size of photos without significantly affecting their quality, freeing up hard disk space. In addition, it hunts down duplicate photos, and deletes them, to save even more space.
Photo Vacuum Packer is exceptionally easy to use. Tell the program to look in a folder that contains your photos. It then looks for duplicates, examining the actual contents of photos, not just file names, to find copies. It lists all duplicates, and will delete them if you want.
This is a good free one. It has won a lot of software awards.