Whenever you try to do a dental procedure in the mouth there is one thing that interferes more than anything else: moisture. It messes everything up. If a composite gets wet during placement, it may not bond as well to the tooth. One way dentists use to control this is a rubber dam. This is a really strange looking contraption that isolates the working area and keeps the tongue, lips, and cheeks out of the way. It does work. I was trained on the use of the rubber dam in dental school and residency and used it often my first few years of practice. After seeing alternative ways of isolation like cotton rolls, I have gotten away from using the dam. The key is isolation. It doesn't really matter how you do it. If you have experience using other techniques, that's fine. Otherwise, the dam is great. There are drawbacks to using just cotton rolls and paper triangles, etc. You have to be pretty good at suctioning the mouth and keeping the tongue away. In the past, I have found the rubber dam is difficult to place in some children, but many tolerate it well. You have to place this clamp on the tooth to hold it on. If you are doing a crown you kind of have to finesse the thing or cut it away from the area around the tooth you are trying to prepare for the crown. If you have a sensitive child, it sometimes may not be possible to place the clamp without the child protesting. Most of the time it works well. I will say that even though it looks weird, it is pretty comfortable.
There seems to be two camps of dentists on this issue. You either love it and use it often or you detest the dam. Well, I say whatever you feel most comfortable with. Whatever you can use to deliver quality care, then us it.