Think about the following scenario: A child is diagnosed as having NF type I, an autosomal dominant disease. However, neither of his parents is affected. This may be due to a new mutation in the embryo, which would explain the son having the disease with normal parents. However, one other possibility is the presence of germline mosaicism - in other words, gametes of a parent having the mutation while his/her somatic cells being normal. In this instance, the child can still have the disease with "normal" parents (but in fact, one parent would be phenotypically normal with genotypically having germline mosaicism).
So what is the significance? It's about the possibility of other siblings having the disease. If a parent has germline mosaicism (instead of the case of new mutation in the embryo), then there is a possibility that other siblings in the future could be afflicted with the same condition.
