Mammals have a set of deciduous teeth. Why?

One role of the primary teeth is to help the permanent teeth come in in the correct position.

The primary teeth help the permanent teeth erupt in their normal positions; most of the permanent teeth form close to the roots of the primary teeth. When a primary tooth is preparing to fall out, its root begins to dissolve. This root has completely dissolved by the time the permanent tooth below it is ready to erupt. (via)

I would also guess that since teeth play such an important role in eating and digestion (and ultimately on an individual’s survival), it is good to have a backup set in case the first teeth became damaged in youth. Probably not a bad idea, seeing how impossible it’s been to get my little sister to take care of hers… so, they can be damaged and mistreated in the juvenile phase, but be guaranteed to have a new, structurally sound set into adulthood.

Sharks have lots of iterations of teeth: Sharks, as well as other Chondrichthyes, have a polyphydont dentition, in which old teeth are shed continually throughout the fish’s lifetime, and new ones are rotated into place on a conveyor belt-like structure.”

Source

And, those of us that had/have only one, two, or zero wisdom teeth are more ‘evolved’ than those that cut the usual 4.

I only had my top wisdom teeth, no bottom ones.

These molars are being phased out right now due to lack of needing the extra grinding power.

My first thought, which isn’t very scientific, was that you need teeth when you’re young. Your “baby” teeth are small, but when they fall out and permanent teeth come in, they’re much larger. Ever notice how kids whose front teeth have come in look bucktoothed?

Your body grows as you get older, but teeth don’t grow. If we didn’t lose our baby teeth, our teeth would be small throughout our lives.

I don’t think it has so much to do with potential damage to teeth when we’re young, as it has to do with the body needing teeth at different ages (and therefore sizes). Makes sense to me.