The reason that stars like Leonardo di Caprio, Julia Roberts and Tom Cruise can command such high salaries is because of the high box office draw that their names can bring to films.
"Films are sold on the basis of their stars, who have a 'marquee value' in opening a film."(S)
Point your mouse at the actors below to find out how much their fee is for appearing in a film. (S)

Films stars such as Tom Cruise or Leonardo DiCaprio have huge followings of devoted fans that are likely to go and see one of their films regardless of any other factor such as genre or special effects. When a studio hires a big name star they are essentially cashing in on the recognition that their name brings by associating it with their film.

...next

For this reason trailers for films in which there is one dominant star name will inevitably focus on that star above anything else. In the trailer for Jerry Maguire (Cameron Crowe, 1997), Tom Cruise is used as the main selling point. His character, Jerry Maguire, provides the voiceover at the beginning of the film in first person narration, and his image is either shown or implied in virtually every shot.

If you watch the trailer you will notice that, when another character is shown instead of Maguire, they are either looking at him, talking to him, or talking about him. Thus every character becomes defined by their relationship with Jerry Maguire. When Cuba Gooding Junior's character is first introduced to us he is talking on the phone to Maguire, holding the infamous "Show me the money" conversation. Renee Zellweger is likewise introduced when she stands and offers to leave the company with Maguire. The trailer is telling us that this is Cruise's film. He holds the title character, and the story revolves around him.