One reason to study green algae is because they can teach us something about the evolution of multicellularity. A number of related species in the Volvocalean family form a gradation of complexity between single-celled and simple multicellular organisms. The members of this family of algae differ in size, the number of cells they produce, and whether or not there is a split between germline and somatic cells. This split is thought to be central to understanding how a new level of individuality has evolved. — Calcott 2011, p. 39.
Calcott, B. 2011. Alternative patterns of explanation for major transitions. Pp. 35–51 in The Major Transitions in Evolution Revisited (Calcott, B. and Sterelny, K., eds), Cambridge University Press.