Lunch interviews tend to be a second interview rather than the first.
The interviewer gets to find out the social skills of the candidate and due to the interview being unstructured and not in an office, an informal atmosphere is created which puts both the interviewer and the candidate more at ease.
This also gives the candidate a chance to ask their questions regarding the job or the company in a more informal setting.
The Interview process is important for both the candidate and the interview. Hence why it is necessary that it is conducted in a way that will suit both parties.