There is a latch on the Speed Graphic bed, near the turn knob.

It can turned left or right, but does not seem to have any function.

Does it have a function?

Technical term: Yoke lock. With the lever foward or parallel to the rails (yoke in Graflex speak) the rails will move in and out easily with the focus knob(s). Moving the lever to the right, as you hold the camera for shooting (picture taking), to its limit it should lock the rails from movement. Depending on wear and adjustment it may just dampen the rail movement or have no effect at all.