It's encouraged to be generous with others.
Elizabeth is a small business owner on a tight budget and is taking a prospective financial backer out to lunch to try to convince him to invest in her business.
Elizabeth wants to make a good impression to the investor.
Elizabeth uses her limited to funds to buy an average cost lunch for them, but that enables her to leave a large tip for the waitress.
Elizabeth uses her limited funds to buy an expensive lunch for them, but this requires her to leave a very small tip for the waitress.
The investor sees how well Elizabeth rewarded the waitress and decides that Elizabeth is the kind of person he wants to invest his money in.
The investor sees how poorly she rewarded the waitress and decides Elizabeth is not the kind of person he wants to invest in.