When a child reaches the stage when their permanent (adult) teeth are starting to appear, it is not uncommon for there to be a crowding issue. This is something that needs to be addressed in order to allow the new teeth to come through into their correct alignment. This can be particularly important for the canine teeth which often do not have adequate room to move into their correct position or orientation.
In the past, this would generally lead to oral extractions in order to allow the canines to form into position. Today Orthodontists would much rather create space than remove healthy teeth. This is where the D-Gainer plays an important role.