Classic screenplays can teach you story structure, they can teach you technique, and they can help you draw conclusions about why certain aspects of story have gone out of fashion, why some remain, and why others should make a comeback.
Classic movies may not have a lot of explosions or crazy car chases, but they still keep audiences nailed to their seats. So: how do they do it? What makes their stories so romantic, funny, heartrending or frightening? Why have these movies stood the test of time...?