There is only a given quantity of resources (nutrients, light and water) in a place. If all the seeds of a plant were to fall at the same place and start growing, they would be competing with each other for the limited quantity of resources. They would also be easy targets for any predators, since they are clustered in a small region. If there was any disease specific to that plant in the area all of these plants would die.
This is why seed dispersal is needed. Having the seeds spread over a larger region provides better chances of survival (against predators, diseases, adverse weather, nutrient limitations, etc.). Dispersed plant populations will have more success in growth and propagation. In the process of seed dispersal, a number of agents (such as wind, animals, human beings, etc.) are used by plants for seed dispersal.
Hope this helps.