Film Promotions – Name Changes

Harry Potter and The Sorceror's Stone Poster (America)The Boat That Rocked - German Promtion

Was in Germany recently seeing family, and I came across a poster for The Boat That Rocked, only with a different, english, title. And it made me think, why do names of films, plays, books etc change when they are released in various countries? I mean, another example, Harry Potter, why was the first film re-entitled Harry Potter and the Sorceror’s Stone in America, what was wrong with Philosophers?

I am curious to find out whether this is the case with other films, anybody know of any other examples like this? Bar film titles that have been translated into different languages, because that is naturally understandable.