The importance of Charley's character during the card-playing scene in Act I of Arthur Miller's Death of a Salesman is that the dynamics between himself and Willy become unveiled, clearly showing Willy's problems and weaknesses, and how Charley is ultimately Willy's foil.
During the card scene the audience witnesses that Willy and Charley's long-standing friendship has had up and downs. Regardless, Charley is Willy's only remaining friend and a true one, at that. Willy relays to Charley his concerns about Biff, and Charley has the trust to openly tell Willy to quit seeing Biff as his plaything; to let him grow up.
After a while, we realize that Charley has been giving Willy money to pay his bills, since Willy really does no longer make a salary. However, when Charley offers Willy a job, Willy becomes insulted; he has gotten to a point where he prefers Charley's handout as a gift rather than working for Charley, as an employee.
This is because working for Charley would signify the end of Willy's dream of superiority over people like Charley. In Willy's mind, it is charisma, quick wits, and good looks that create the man. Charley, in his view, would be a "loser" because he is not over-the-top, has always led a quiet life, and lives reasonably within his means.
Although it is clear that Charley, as well as his son, Bernard, are the true success stories of the American Dream in Death of a Salesman, Willy stubbornly sticks to his misconstrued ideas about success. Willy represents the fake ideals that have consumed the psyche of the American society, while Charley is the traditionally-minded man who works to achieve however much he can. These have been the dynamics between Charley and Willy for decades. This is the reason why Charley finally speaks clearly to Willy and says that Willy has always been jealous of him (Charley) his entire life.