or the designer couldn't do the sensible thing and stretch the wing.
In some specific cases, he just MAY not : standard class and 15m class gliders must have a wingspan less than or equal to 15m, routine checks are performed in competitions.
Another positive effect of the winglet is to reduce the spanwise flow, leading to much nicer low-speed handling and stall characteristics (Discus 1 : stall speed reduced by 5kts).