Sheaths, holsters, wallets, belts, lots of stuff, you can get a ton of benefit out of using a clamping mechanism.
What exactly is it you are making? Most 3d objects out of leather start off flat. Not a lot of cubes occurring out of leather without a few flat pieces being sewn together somewhere.
You really need two hands to sew leather well. Having something to hold them makes life a lot easier and your work a lot more tidy. Eliminating variation in your stitching will become a goal here shortly. Clams, ponies, and horses make that goal far easier to approach than freehanding it.