Any time a business runs a low cost special to get you in the door they hope to grab new customers that will return again and again. And it must pay off otherwise they wouldn’t participate in the future.
An example up here is that a lot of the Niagara wineries will participate in wine festivals, where you basically pay only $10 per winery (sometimes less!) and that gets you one glass of wine and a small food pairing. They make nothing on that (maybe even lose some money) but they hope that you will buy a few bottles of their wines and/or become a repeat customer in the future. When my girlfriend and I recently visited 9 different wineries under a festival like that, she bought wine from 4 or 5 of them and she wants to return again to some of them. And another factor is word of mouth too. When businesses open their doors and people have a good experience, they tell their friends about it.
On the flip side, if a restaurant treats these customers with less respect because they’re not making any money from them, they run the risk of getting bad mouthed. So they need to have just as good customer service as they would with a customer who is paying full price. You never know how much business you might be losing when you treat someone “lesser” on these special events