I have a question about one of the Algerian Arabic lessons (Basic Prepositions)

Could you explain why we must put “raha” E.G.

il-bent raha fil tomobile
the girl is in the car

why can’t we just say:

il-bent fil tomobile

Could you explain the system behind doing this as I understand its different for different nouns e.g.

l-kiteb raa fwq il-beeru
the book is on the desk