If the deciduous teeth are eventually replaced by permanent teeth , why do you get them in the first place?

Small mouth: When we are children our mouths are small and adult teeth would be crowded. As the mouth grows we need bigger teeth to fill it.

Proper development: Teeth erupt in a specific sequence to allow children to develop bones, muscles, and speech normally. Teeth are important in not only eating and learning speech, but in the normal development of the face and jaws. Any changes in this sequence by early loss of teeth, or untreated dental infections, can have serious and long-term effects on a child's physical and mental well.