Are you asking or are you telling the answer? Its quite obvious that Bernard Shaw used 'rain' as a plot device to introduce the characters to the audience. Plays normally go by a logical progression of story, everything has to fit in place. Since it was an introductory scene it serves an important purpose to create relevance with the rest of the story. Besides, for later developments it gives a nostalgic value to the audience as they recall how events proceeded from that scene onwards.
