Why does algae multiply in different ways, what are the benefits of each of these methods?

Algae reproduce asexually and sexually. The advantage of these methods is that under both favorable and unfavorable conditions, algae have the ability to reproduce their own kind.
Under unfavorable conditions, they reproduce sexually: gametes appear in the cells of the algae; then these gametes merge in pairs and form a zygote, which is at rest until the onset of favorable conditions. When these conditions arise, the zygote divides into spores, which then germinate and give rise to new algae.
Under favorable conditions, they reproduce asexually: the cells of the algae either divide in half, or some part is separated from the parent alga, which forms a new alga.