The side plates of the splitter prevent air spillover on the ends of the splitter which increase its effectiveness greatly.
These perform two duties, one they seal the sides of the splitter to the ground and two, they act like a stop to help keep the splitter height above the ground more constant. This reduces the chances of pitch causing the front of the splitter to hit the ground, cutting off the airflow and greatly diminishing the splitter's effectiveness.
The side plate skids are a consumable item and are inspected every time the car goes out and replaced when needed. The splitter is designed to hinge in relation to the chassis so when the skids hit the ground the splitter can pivot and not be excessively stressed.