Stars are performers in a film whose names are internationally known and whose performances are globally enjoyed. Without getting too deep in film theory, stars are basically tools used to sell films and are in this sense, used as a commodity, something which can be bought and sold. For example, Julia Roberts' name is often used to sell romantic-comedy films, and Arnold Schwarzenegger is often the main pulling power for action/adventure movies.
The movie going audience gain dual profit from going to see stars in a film, in the form of pleasure and profit motives.
Profit/financial motive means that actors or performers in a film can be seen as commodities with exchange value (they can be bought) and use value (they can be sold). This goes hand in hand with their...
Pleasure/semiotic motive, based on the image of the actor (ie. Pierce Brosnan as a James Bond style action leading man). This means that the audience gets pleasure from watching their favourite stars, making us want more. More often than not, we can relate to and identify with characters portrayed by the actors onscreen, thereby giving us a kind of semiotic "value for money".
Profit motives feed back into future performances, making the audience return to the cinema (and so adding to the pleasure motive), wanting to see more. For example, if you liked the last film Jim Carrey was in, chances are you'll go back to see him in the next one!
Here's the theory...
Feminist theorist and critic Laura Mulvey with her obsession for voyeurism and sexism in Hollywood, claims that we are attracted to starts, that we get pleasure from watching them onscreen, because via their performances, we can act out our own fantasies. The narrative style of the film "manages" or contain our desires, representing one of the ways in which Hollywood forces upon us the spectator role, consumers of the product, by drawing audiences in with the promise of unfulfilled desires which are played out on the big screen in the safety and secrecy of a darkened room.
Why else would you want to go and see trashy teen slasher flicks like Wrong Turn and Jeepers Creepers 2... ;)