The lunch buddy doesn't get the option to enter feedback for a candidate and often doesn't even get invited to the interview debrief. They really are there just for lunch.
Source: have been a lunch buddy several times.
Currently an Amazon employee. 40+ interviews + debriefs. Not once has "what did the lunch buddy think?" ever been uttered.
I've been a lunch buddy myself a handful of times. My goal is to have a good time, answer any questions the candidate has, and squeeze as nice of a meal (+ coffee) outta the process as I can. I've never been asked for feedback
I think it is worth it to the company for me to spend the 5 extra minutes of my time (since I'm going to eat lunch anyway) and $25 on lunch to defuse those concerns and sell them on the team. With the phone screen, the interviews, the feedback forms and the debrief, the company is spending a ton of time on each candidate brought on site. Loosing a candidate you make an offer to is the worst waste of time.
And what do you tell them / what do you think?
Usually they will tell you whether lunch is also an interview or if it's just lunch. From my own experiences, Microsoft typically does a more touchy-feely interview (still a real interview) over lunch, but Google typically doesn't. With the tech giants, you can take this at face value.
That doesn't mean your lunch buddy will be a good conversationalist though. Mine mostly were not (including at AirBnB, but I only point that out for the irony against their stated cultural values). I would sometimes just ask for the last 20 minutes to meditate.
Also that's not an insult or a judgement. Being a good conversationalist in professional settings is hard, and interviews doubly so because interviewees are nervous for good reasons. So I don't judge engineers or even PM's by how good at bantering they are during interviews, because it's not their job to be good at it.
[edit: Wow, so much down votes.
I have interviewed and been interviewed at multiple big tech companies. In all cases the lunch has been part of the interview process, in most I’ve even been given an explicit “this is X, they’ll be talking about Y over lunch”, or it’s been called “lunch interview”.
If the company isn’t treating the lunch period as an interview you’re wasting your engineer’s (or whatever) time, and the candidates. ]