In airplanes like the Big Stik, UCD etc that dont have the winglet, the triangular wingtips do the same thing- they prevent the air leakage from bottom to top. They also serve another purpose if I am not mistaken- the triangular tip generates a vortex that gives a huge amount of lift for a very short time at a high angle of attack. They also create a very stable flying airplane. Thats the reason why many classic airplane designs- UCD, Stik etc have them. If you break or remove them, the minimum landing speed will increase. At a minimum, they will cause a roll effect if there is a differential. Dont ask me how I know

Ameyam