Health:
A breed such as a Cane Corso has very thick, heavy ears that hang down. Even with thorough cleaning often, moisture gets trapped inside the ear and it is also very dark and warm which is the perfect breeding ground for bacteria which causes ear infections. A nice working crop allows lots of air flow keeping the ear dry and cleaner resulting in fewer if not no ear infections.
Safety:
I plan on in the future once I get my farm/ranch, having Caucasian Ovcharka and Central Asian Ovcharka as livestock guarders against wolves, coyotes, wold dogs and anything else that would like to try to harm my animals. Floppy ears are a perfect "handle" that when ripped makes a bloody mess when fighting off other canids. Cropping the ear means nothing for the wolf or coyote or whatever to grab on to.
I also plan on having a PatStaff or a PatStaffJag as a verminator for the barn buildings of my farm/ranch. These lil guys will be up against anything from mice to rats to coons. If you have ever excountered a coon, you will know that they can fight back very nasty when cornered. Again the ears are just something for the coon to bite and tear making a painful bloody mess for the dog. A good working crop stops this from happening and the dog is able to rid the barn of the pesky rodents a lot easier!
Need me to go on?