Why did they use chemicals and glass plates in early photography?

until celluloid was invented Glass plates were used because it was the only way to suspend the emulsion evenly and allowed light to pass through. It was easier to handle.

Some glass plates were large and were put into contact with paper to create a positive image. Chemistry was used to convert the silver halides in the emulsion to visible black metallic silver halides. When light was shone through the glass the black metallic silver halides stopped the light and the paper was exposed where the light pass by the halides. This caused the material being exposed to show a positive image.