Dining with a prospective employee allows employers to review your communication and interpersonal skills, as well as your table manners, in a more relaxed (for them) environment.
Table manners do matter. Good manners may give you the edge over another candidate, so, take some time to brush up your dining etiquette skills.
The employer may be trying to determine if you are a good fit socially.The employer may ask you to bring your significant other, spouse or fiance. This is usually on a second or third interview.