These collections of stars by brightness and temperature are important when talking about stellar evolution. In general, stars are created in the main sequence. (Of course, when we say ""in" the main sequence" we really mean "having a brightness and a temperature that causes them to be plotted within the main sequence on a Hertzsprung-Russell diagram.") After billions of years, they expand into red giants. Then, after another one or two billion years, they shrink into white dwarfs.