Baby teeth (or primary teeth) play a very special role in appearance and function (speaking and eating) just like the permanent adult teeth. But they also hold space for the unerupted permanent teeth. If lost early, space can be lost, causing possible permanent teeth crowding almost ensuring orthodontic care (braces) later.

So, obviously, cavities in baby teeth should be attended to as soon as possible, although it would be preferable to prevent cavities altogether.