It is for creating halftones from continuous tone subject. It is placed in contact with the film (or photosensitive material) as a sandwich using a weak vacuum to maintain contact. You could use it for contacting a negative, or expose in a process camera. It would be "hard" to use in a regular camera because it would be hard to make a sandwich with the screen in close contact with the film.
85 lines is relatively coarse, newspaper grade, suitable for a hand-coated high-contrast light sensitive material. Great for experimenting because it will give successful (at least interesting) results even if your material is crude or the contact is imperfect.