parshanut torah comment - Why the extra words "who sits on his throne", "who is behind the millstone"?

In Parshas Bo, 11:5, the Torah says:

"Every firstborn in the land of Egypt shall die, from the firstborn of Pharaoh who sits on his throne, to the firstborn of the slave-woman who is behind the millstone, and all the firstborn of the animal".

We know what kings do; they sit on thrones. We know what slaves do; menial labor.

Why does the Torah use these extra words ( in bold above)?
What do we learn out from them?

(The assumption being that the Torah is exact in every way and every word can be learned from.)