Why do many unicellular algae, like Chlamydomonas, have organelles of movement in water

Why do many unicellular algae, like Chlamydomonas, have organelles of movement in water, and all large plants are attached to the substrate?

Algae, like any plant, need light for photosynthesis. And the main condition for photosynthesis is light, so unicellular algae should be closer to the water surface, where there is more light.
But water does not stand still, but moves by the wind, waves, currents, so many unicellular organisms have organs of movement that are needed to move to illuminated places.
Large plants are multicellular, longer and heavier. To prevent them from being washed away, carried away by the current, they must have an “anchor” – attachment organs – rhizoids.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.