4 year old has dermatomyositis ( rare , 1 in a million for juvenile males); an immune disorder. He has been getting meds and can walk again. However a side affect of meds is that he has 8 cavities. The dentist says at least one needs a crown. I expect he will need more before long.
The reason that dentists might want to crown a baby tooth instead of removing it is to prevent crowding of the jaw from premature tooth loss, and to prevent the decay spreading to adjacent teeth.