Danny, I'm sure that a media luvvie would be able to explain it in a thrice, while simultaneously suggesting that we are uncultured clods for having to ask. No doubt it adds dramatic effect to what would otherwise be the visual version of dead air. Yesterday I was watching a programme about Alan Turing which had him walking around in filmed scenes as a cartoon character, again with no clear reason for doing so. I think you have the nub of it when you mentioned CGI, in other words because it can be done it is done!
As it is from the BBC, I suspect also that they would baulk at the programme being described as a Scientific Documentary. They don't tend to do scientific much unless there is a strong social or political message that can come out of it. So were the really important ancient Egyptians not the ruling class but rather the PBI? Discuss...