Periodontal disease affects over 80% of the population. There is no cure for it, but it can usually be stabilized with good oral hygiene and regular dental hygiene visits.
It is critical that the areas of periodontal disease be meticulously cleaned by the patient. Often, the teeth are so crowded that it is almost impossible to keep them clean. In these cases, orthodontics (straightening the teeth) will move the teeth into position where they will be easier to maintain and clean. Almost always, the teeth with periodontal disease are the hardest ones for the patient to keep clean. I think your dentist has made a wise suggestion.