Seeds are dispersed by different methods. One of them is wind dispersal. Some don't need much care, effort and nutrients. These seeds when get dispersed, travel to many places. When some of those seeds fall into the garden they start growing or germinating. Then the gardener has to do extra effort to clean the garden. Sometimes the seeds that get dispersed are the ones you want to plant. Then the garden has to take care of that plant also. It may require extra care to grow and may need more nutrients. So the gardener will have to put in fertilisers and manures. Thus the dispersal of seed ( which you called many ways to spread seeds) can cause problems for the gardener.