Charley and Willy are next door neighbors and antagonists, but at the same time, they are friends or, more precisely, frenemies. They play cards together, and Willy is dependent on Charley for a flow of "loans" that keep him financially afloat. Charley, who owns a successful business, offers Willy a job when he is laid off, even though, as Charley puts it:
I know you don’t like me, and nobody can say I’m in love with you
The play makes clear that Willy is the problem in this relationship. Charley is a kind, humble, helpful, down-to-earth person, but rather than appreciate those traits, Willy is eaten with jealousy at his friend's success. Rather than being grateful for the lifeline Charley offers him, Willy bitterly resents him.
Because we are so much in Willy's head throughout the play and see life largely through Willy's eyes, Charley is a crucial corrective to Willy's delusions, which makes their relationship important. Willy constantly looks down on Charley for lacking the flash and personality that Willy respects, but what we see is that Charley has the traits that actually make for a successful businessman: kindness, insight, commonsense, and humility. In fact, through the fraught relationship, we can see exactly how Willy alienates the people around him. Willy always has to win, always has to be on top, is always bragging and glad-handling people and being aggressive. All of these are behaviors that alienate others; despite Willy's delusions, the way he behaves is not conducive to a successful sales career. If Willy can't even get along with Charley, it is unlikely he is going to make many friends.
Charley is one of the very few people who shows up at Willy's funeral. He shows generosity and empathy towards his neighbor, traits that Willy would have done well to emulate. Charley models the lack of competitiveness that Willy would have benefitted from learning:
Willy was a salesman. And for a salesman, there is no rock bottom to the life. He don’t put a bolt to a nut, he don’t tell you the law or give you medicine. He’s a man way out there in the blue, riding on a smile and a shoeshine. And when they start now smiling back—that’s an earthquake. And then you get spots on your hat, and you’re finished. Nobody dast blame this man. A salesman is got to dream, boy.