Would this be to control the stretch during the fist few weeks of flower? I've been toying with the idea of keeping veg lights on my plants for the first few weeks of flower as I believe the bluer light keeps the plant shorter.
^^ I think this is because MHs have more blue and less red, so they contribute to continued vegetative growth at the expense of bud growth. This is the same reason people switch fluoro lights from 6500k in veg to 2700-3000k in flower.