1. They prevent spanwise flow under the 1st wing element - improving Cp underneath the 1st wing element.
2. Due to the spanwise pressure differential across the wing, the trailing edge will result in some rollup (hence: vortices). These will help ensure attachment on the 2nd wing element - more downforce on 2nd element and less wake effects for the diffuser later. The vortices will long since have burst by the time the flow reaches the diffuser.
3. These vortices will be directed toward the outside of the wheel; their presence should help guide more flow straight out beyond the wheel as well. Particularly when there is a steering angle on the outer wheel. This means yet more downforce from both 1st and 2nd wing elements.