When I worked at a company that offered free lunch, the company used it to help build rapport and camaraderie among the coworkers. It's one time during the day when the entire team can sit down and eat lunch together and get to know about each other, which is not possible through Slack nor via meetings. Just hanging out with everyone on the team, talking about the latest Marvel movie or Star wars, etc. It was also about convenience as well as reducing stress and decisions on an already-busy day.
If the team still wants to do lunch together everyday at an outside restaurant, it would not be easy, especially the logistics, which would be a nightmare -- getting everyone to agree on one place where everyone likes their food, finding a place that can seat the entire group at a single table, finding transportation / assigning driver/riders if it's beyond walking distance, and having to divide the check afterward (or Venmo), etc. Just for a meal that lasts no more than an hour, top.