When the movie itself begins, there is usually an initial 'cold' period.
However, a strong main title or sequence can be sufficiently provocative and entertaining to induce the audience to sit down and 'look and listen', because something is really happening and communicating with them on screen.
I believe at this moment it is possible to project a symbolic foretaste of what is to come, and to create a receptive atmosphere that will enable the movie to begin on a seamless and higher level of audience participation.
The end of the movie when the credits start is again another chance to graphically communicate and reintroduce the visual style, which is unique to each movie. Like a book cover has a front and back cover.