The biggest functional difference between a leaf shutter and a focal plane shutter is the ability of a focal plane shutter to precisely allow the same amount of exposure time for the entire field of light collected at the front of the lens and to allow the practical use of faster shutter speeds.

Due to the fact that leaf shutters are open in the center longer than at the edges, the light coming through the center of the lens falls on the image plane for slightly longer periods that the light coming from the edges of the lens. This wasn't such a big issue when photography first got started and the emulsions were so low in sensitivity that typical exposure times were in minutes, rather than hundredths or even thousandths of a second! In fact, the first "shutters" were lens caps or plugs that were removed and replaced on the front of the lens by hand.

As cameras became more sensitive to light and the desired exposure times got shorter and shorter, the limitations of the leaf shutter became a more significant issue. Even so, there are still new digital cameras produced today that use leaf shutters. The designers feel, and the marketplace seems to agree, that the tradeoffs are worth it in some cases.

A focal plane shutter can be designed to begin exposure on one side of the frame and end it on the other side of the frame. This allows all parts of the frame to receive light from all parts of the lens for the same amount of time. The earliest single curtain focal plane shutters, such as those used in the Speed Graphic, had a fixed slit that passed across the focal plane. By allowing the user to select different slit widths and spring tensions for the mechanism that drove the slit across the focal plane, shutter speeds ranging from 1/10 second to 1/1000 second were possible using most of the various models of the Speed Graphic.

Why would the Speed Graphic have both a focal plane and leaf shutter? It doesn't necessarily also need a leaf shutter. Barrel lenses without a leaf shutter can be used with a Speed Graphic. The focal plane shutter is used for speed, specifically faster shutter speeds, thus the name Speed Graphic. But the camera was certainly not speedy in terms of shot to shot intervals and the operation of the FP shutter took longer to manually reset the FP curtain between shots than the operation of a leaf shutter in the lens. This may be one reason many users preferred both options. The lineup of lenses that included leaf shutters offered by lens makers could be used across both the Speed Graphic and the Crown Graphic and Century Graphic models. (The lack of a focal plane shutter allowed the Crown Graphic to be made slightly thinner which allowed use of some wider angle lenses than could be used with the Speed Graphic.)

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Though not exactly applicable to your specific model, here is a link to the instructions for a c.1925 Top Handle Speed Graphic.