The big misconception would be “It is only baby tooth and I don’t need to get any work done on them”. This is a big misconception commonly people have. Front four baby teeth (the incisors) top and bottom falls out around age 6, and around age 12, most of remaining baby teeth (the primary back molars and canine) fall out.

If you end up losing the primary (baby) tooth too early, it can cause a severe misalignment of your permanent dentition such as crowding. Baby teeth need to be maintained long enough to maintain the space so that permanent teeth have enough space to come in. If you end up losing the baby tooth too early, space is closed and its successor permanent tooth would not have enough space and it can cause the permanent tooth to be erupting either front or back-side of the dental arch. Not only that, typically the trend that people have with baby teeth continues onto permanent teeth; meaning if you get a lot of cavities in baby teeth more likely you will get a lot of carries on permanent teeth and if you don’t take care of caries on baby teeth, you will more likely not take care of carries even on permanent teeth. Therefore, it would be wise to start taking care of baby teeth as well.

So why not just leave the baby teeth so it can maintain the space?

Cavities are caused by bacteria. In certain situation, it may be okay to leave the tooth and let them fall out on its own but, most cases, you would need to get work done even on the baby teeth. Bacteria cause infection in the mouth, and oral infection may cause fever and other bodily infection.

References:
https://www.mouthhealthy.org/en/az-topics/b/baby-teeth
https://www.aapd.org.au/post/Why-it-is-important-to-fix-baby-teeth#.XN8QY8hKiUk