Why do we have baby teeth if they're just going to fall out anyway?

Available Space: An entire adult dentition wouldn't fit in an infant's mouth, right? Primary teeth are smaller and are limited in number as to fit in a child's mouth. Evolution at work.

Simple answer: The permanent (adult) teeth are too large to fit into a baby's mouth until the jaw is fully grown. That first set of teeth (baby teeth) are good enough to get it through the growing stage of its life. Most mammals go through the same, two set, process. Hope it helps.

Permanent teeth start forming in the jaws during the 3rd trimester, and the first ones are not ready to erupt until about ages 6-7. The 32 permanent teeth are too large to fin into a 6 month-old's tiny mouth. For that reason the 20 baby teeth are wonderful place-holders and allow children to eat solid foods until about age 12-14 when most permanent teeth (except wisdom teeth) have erupted.