Singing in the rain film

At the beginning of Singin’ in the Rain (Kelly and Donen, 1952), Don Lockwood (Gene Kelly) tells the story of how he and his friend Cosmo (Donald O’Connor) worked their way to Hollywood. The audience experiences this story as a flashback. However, this flashback serves a purpose other than merely telling the story of Don and Cosmo. Much like Singin’ tells the story of the development of sound in film, this flashback tells the story of early years of cinema. Describe how this flashback shows the development of cinema in its early years?