Why do boys have hair on their chest?

No one knows exactly why (some) men grow chest hair, but it is something called a "secondary sexual characteristic" that appears after puberty, like facial hair and other body hair, and in men along side a deepening of the voice, etc. It is originally probably simply a way to attract a mate, not really for any concrete purpose other than to attract attention. Human gene pools that developed in areas plagued by lots of insects, etc., tend to be hairier as well--so that's part of it, too.