A vortex is generated because when a wing produces lift, you have high pressure below the wing and low pressure above. They will try to equalize at the wingtip, combine this with relative wind and you get a swirling motion. At low speeds, because of higher AoA and higher camber (from high lift devices), this pressure difference will increase and create more powerful vortexes. A winglet stops this motion by effectively being a barrier/wall so that the pressure differences cannot be equalized.
On a 737, winglets generate a 2% fuelsaving. For a big company with small margines, this can be the difference between earning money or losing money.