as a breeder of danes i dont believe in it akc has relaxed the rules on this because there is no reason for it but looks cropping is a very long drawn out process and it doesn't always work i always ask buyers not to do this because its painful for the pup and the months and months of care is very difficult
The most common reason was originally for fighting. Dogs with cropped ears couldn't be hurt or stopped by their opponent if they tried to grab their ears. Breeds that used to (and still are, sadly) be used for fighting, such as AmStaffs and APBT's are cropped for this reason. Doberman Pinschers for example were cropped because they were originally a guard dog.
Either for looks or for fighting. Sometimes they can't get papers on the dog without cropping ears or tails for that breed of dog. Personally I don't see a problem it as long as a vet is involved with the procedure.