There are a number of reasons why you may need to learn how to use dog diapers:

Urinary incontinence: This could be caused by a number of health issues and is sometimes a problem as dogs age and lose muscle control. Incontinence should not be confused with behavior-related lapses in house training, and could be a sign of illness, including a urinary tract infection or diabetes. If your dog has a lot of accidents, you should take him to the veterinarian as soon as possible to check for health issues and get him proper treatment. In the meantime, a diaper can protect your rug and furniture and give you peace of mind while you and your vet work to get the problem under control.
Illness or injury: Some dogs are unable to go outside or stand to relieve themselves while recovering from an illness or injury. A diaper can help make your dog's recuperation more comfortable for both of you.
Elderly dogs: As some dogs age, they lose the ability to control when they go. Other elderly dogs have difficulty standing or going outside due to arthritis and joint pain. Another fairly common ailment in senior dogs is canine cognitive dysfunction, a condition similar to Alzheimer's in humans, which the Massachusetts Society for the Prevention of Cruelty to Animals notes can cause a dog to forget his house training. A diaper could help prevent accidents in any of these cases.
Medications: Some medications can cause drastically increased urination, writes the Tufts Cummings Veterinary Medical Center. One example is furosemide, a diuretic commonly used to treat congestive heart failure. Rather than trying to keep up with your dog's frequent urges and making sure he goes where he's supposed to, a diaper might make life easier for the two of you.