As Alanna said, the main function is to look pretty. For example, the pretty girls in beer commercials are "eye candy." In some commercial trade shows, pretty girls are hired to hand out company brochures. They aren't employees and they don't know anything about the company's products. They are "eye candy" to get male customers to visit the booth.

Candy tastes really good. "Eye candy" looks really pretty. It's like candy - but for your eyes. (Also, sometimes, there is a secondary comparison that the same way candy doesn't have nutritional value, "eye candy" also doesn't have any value except looking pretty.)

It's typically used when a man or a woman finds someone or something attractive, usually someone though. Typically, its it's a slang way of saying they like what they see.

Eye candy usually refers to a person who is attractive. We usually use the expression in a context where we want to emphasize the main function of this person is to be attractive.