Spores are reproductive cells that can produce new individuals without fusion with other reproductive cells. Like spores, the function of seeds is to produce new generation of plants and so ensure the survival of the species. But seeds accomplish this task much more efficiently.
Spores leave a great deal to chance. Each one consists of a single cell that contains little or no food reserve for the new plant. And it can germinate and survive only if it happens to land in a place where conditions are just right for growth. As a result, mosses, ferns and similar plants must produce spores by the millions to overcome the great odds against their survival.
Seeds, on the other hand, give the next generation a head start in the struggle to mature into new plants. Each one consists of many cells within a protective covering. The cells moreover are usually organized into an entire embryonic plant, one that is complete with rudimentary root system, stem and leaves. And in almost all cases, the seeds contain a food supply that supports the emerging plantlet until the seedling can exist on its own. Seeds are much efficient than spores, in fact the plant that bear them have become dominant vegetation on earth.