Why do biologists classify red and green algae with land plants?

Red and green algae are photosynthetic and are thus autotrophs. Otherwise, they are aquatic and (in the case of green algae) can be unicellular. But these are similarities that are not sufficient to define algae as true plants. All plants in the Kingdom Plantae are multicellular and terrestrial (ancestrally terrestrial in the case of waterlilies). Green algae are important in the study of plants as they show the base of the plant kingdom, hinting at what a common ancestor to the whole kingdom may have looked like. In particular, the charophytes are probably close to the common ancestor of all land plants. Thus, in the study of land plants, green algae can be considered the most recently diverged outgroup. And, earlier still, red algae diverged.