It is true that baby teeth will fall out, yet there are very good reasons to have them filled when caries (cavities) are present.
Any tooth which has a cavity that is not properly fixed can eventually cause a tooth ache. It can also cause infection in the surrounding gums or bone. The earlier that a cavity is detected and a restoration (filling) placed, the lower the likelihood that this will happen. Infection can be dangerous and can also, in certain circumstances, cause problems with the adult tooth that is developing in the jaw bone.
Also, having untreated cavities increases the amount of cavity forming bacteria in the mouth. This makes it more likely that the adult teeth coming into the mouth could develop cavities as well.
So why do we not just pull the tooth? Baby teeth fall out at different times and some of the back ones may not come out until 11 or 12 years of age. These teeth function in chewing but also keep space for the adult teeth to come into place in the proper position.
Your child should be evaluated by a dentist to assess his or her teeth, as well as risk factors for developing cavities or other problems. Your dentist can answer any further questions that you may have.