High risk people . The reason some people need xrays more often is that they are at higher risk for cavities or gum disease. Xrays are only required for diagnosis and it is better to catch and treat problems early than wait until symptoms arise.
Caries Risk. Frequent dental radiographs are taken on high caries risk people. You want to locate and restore any carious lesions as early as possible to prevent larger cavities from forming and destroying your teeth.
Depends on patient. . If a patient is cavity prone every 4 months may be necessary. If someone has a history of strong healthy teeth then every 6 months to two years would be sufficient for that patient. Usually someone on a regular schedule for recalls would get cavity disclosing x-rays every six months. I recommend full mouth x-rays every 3-5 years. Your dentist can help determine what is best for you.