Suppose that there is one plant , it gives ten fruits at a time and these ten fruits have 20 seeds inside them and when they explode the surrounding area would be full of 200 new plants and then these repeat the same process . After some days the seeds would be near 10000 but the space there would be not enough for every seed . And also only one area would have that specie. But if it gets dispersed all regions can enjoy the benefits of that and the seeds would grow properly as they would have enough space that need. This is the main need for dispersal of seeds.