All protists can reproduce asexually. Unicellular protists do so by mitosis. Multicellular protists often produce spores.
Many protists, though not all, can also reproduce sexually. Sex affords faster adaptability by diversifying genotypes, increasing the probability that at least some of a population will survive the stress of environmental adversity. Reproductivity flexibility itself confers adaptability to live in different habitats.
Brown kelp, a common large seaweed, produces spores (a sporophyte). A spore develops into a microscopic life that attaches to submerged surfaces, often in deep, dark waters. These microscopic bodies produce male and female gametes (a gametophyte). Mating of male and female gametes results in a zygote that grows into a kelp that lives near the surface: a photosynthesizing brown alga. Hence brown kelp has a dual life cycle involving both asexual and sexual reproduction: alternation of generations.
One of the protozoan parasites that causes malaria, Plasmodium falciparum, has a complex life cycle involving multiple forms, some reproducing asexually, other sexually. The genetic implications of sexual versus asexual reproduction in these protozoa are not yet understood.