Straight fletching clamps are just that, they're straight. You can usually adjust the angle used to fasten fletching to the shafts, either to the right or to the left. This would be referred to as "straight offset".
Helical clamps "wrap around" the arrow shaft. They apply the fletching around the shaft, so to speak. If taken to the extreme, your fletching could look like the flighting on an auger, or threads on a screw. This allows for more angular fletching attachment. If you tried to match the extreme angles of offset with a straight clamp, the front and/or rear of the fletching would be hanging out in the air, and would not be fastened to the shaft. "