It's needed simply because it's the way that expression works. Try not to think about the words separately. Both words must be used together to mean "about" (such as "mais ou menos" or "à volta de", by the way). The sentence would be incorrect if "de" was missing.
Does anyone know why the ¨de¨is necessary here? Is it linked to the ¨cerca¨. I had thought that ¨They are approximately ten girls¨would simply be
¨Elas são cerca dez meninas¨?
after = depois (DE), before = antes (DE). ABOUT = sobre, ao redor de, cerca de.