It's because your jaw will change shape when there is a void an it can affect other teeth in the row. Also if you remove a tooth on the lower jaw, the corresponding tooth above on the upper jaw can start to travel downwards in the absence of a bite.
Those are valid reasons to avoid having a gap instead of a tooth, but should make no difference in the heavily-repaired-but-"real" vs implant tooth question that the parent poster had.
