What is the purpose of a title sequence and film opening?

The purpose is to hook in the viewer, interest them, and set the mood.  This also sets the theme for the rest of the film and establishes the shot. They can also use sound to help achieve this purpose, for example if they build up tension and then silence - creating a powerful effect.

Also, it credits the actors, producers, directors, crew, distribution and production companies (via ident's). These - in this example, a distribution company - could have a cult following, which would attract a secondary audience.