A lot of old-timey saloons would just put out a lunch buffet and as long as you bought one drink you could take what you like. Pretty good food by some accounts.
I talked to my Dad and he claimed this was still a thing in a few places as late as the early-80s, there were places with “happy hour” where they would put out a small buffet like once a week. He said in college he’d go to them and buy one beer and fill up.
I believe it’s what start ups today would call a “loss leader”. An actual loss (ergo, an actual free lunch), with the expectation that it leads to more sales and profit in the future.