Many years ago, in 1992, I was a member of the Chicago Miniature Schnauzer Club. At that time I had a show quality dog. It was common to keep an eye on the dogs teeth as their bite was developing. The claim was that sometimes the baby teeth do not fall out by themselves as the adult teeth are coming in and it can affect the dogs bite. So breeders and people with show dogs and their vets paid close attention to this because the bite is looked at in the ring.
I suppose that is it possible that this has now become a common practice for Vets to prevent or head off bite issues that may occur. I would think the easiest way to find out why a vet would do this is to ask them.
Although we knew that Roo would not be shown we keep an eye on this because we did not want him to develop any bite issues. I probably kept on eye on his teeth out of habit.
If I remember correctly Roo's teeth were not completely in until 8 months. Of course this may vary from dog to dog.
Jason