The reality is, we don't know! Some children, in retrospect, don't end up needing anything before all their teeth come in, and they would have been just fine waiting until age 12 or so when all permanent teeth (on average) tend to erupt.However, some children have issues that either can't be fixed at all once they are older, or the problems are then much more severe or the results will be compromised.
Here is an example of a child that was alerted to some strange tooth eruption, but the problem was already quite severe. Had the child been referred earlier, the problem could have been much less severe