[Graflex] In its time, what was the purpose of the focal plane shutters on the Speed Graphics?

Sorry if this is a dumb question: But every time I see a Speed Graphic with a focal plane shutter, they have a lens with a Copal/built-in shutter? I understand that you can adapt all kinds of lenses today, hence the appeal, but why back then? All the stock lenses I've seen have shutters coupled with them. 🤔 Many thanks!