It's not really about saving money on waiters - it's more of a mixed situation. On one hand, prepayment through an app actually does reduce the workload on staff and eliminates the hassle of settling up after service, especially when there are lots of customers at once. On the other hand, for cafes it's mainly insurance: they're guaranteed to get paid and won't have to deal with counterfeit bills or people who "forgot their wallet."
Plus, during and after the pandemic, contactless payments just became a habit, and payment systems with integrated processing got way simpler and cheaper. Now it's not some exotic thing like fast food places do - it's the standard. Whether that's okay depends on what you prefer, but technically it's already become worldwide practice :)