It could be an incentive. Some customers are more valuable than others and using percentages means the employees will be extra nice to high-value customers and

It could be an incentive. Some customers are more valuable than others and using percentages means the employees will be extra nice to high-value customers and not waste too much time on the low-value customers. Of course they should be nice to everyone, but if it is a busy day and they have to choose, the restaurant owners would prefer that they prioritize the ones who spend more.