Because vegetation, like grass, trees, shrubs, etc., protect the earth below from outside elements. Roots from these vegetation keep the soil on the ground. Without trees there is an increased chance of mudslides, flooding, erosion, etc.
The Groins are hard structures and used to maintain uplift beaches as to restrict longshore sediment transportation form tasking place. Are meant to capture sand transportation by the currents.
While the jetty is another type of shore perpendicular structure that is placed adjacent to the tidal inlets and controls the inlet migration. Both of them are used to destabilize coastal systems and disrupt the natural sediment regimes.
