Managers, lawyers, graphics designers, salespeople, the list goes on.
For salespeople, sums, differences, multiplication and division of fractions might not be the best example; calculus might be more appropriate. Some of the best salespeople I've ever worked with could effortlessly modify the deal numbers in their head on the fly during a negotiation. For example, it could entail working out a seemingly big "cash discount" number and offering it on the spot, knowing full well that it won't take so many points off the margin that the pricing team will throw a fit nor impact their commission such that they care. These are fairly simplistic one or two decimal place fractional arithmetic that are only moderately more complex for the layperson than tallying up the tip at a meal, but it puts a lot of pressure on the customers when they are told, "if I have to walk out of this meeting without closing, I won't ever get the authority to offer that discount again".
For salespeople, sums, differences, multiplication and division of fractions might not be the best example; calculus might be more appropriate. Some of the best salespeople I've ever worked with could effortlessly modify the deal numbers in their head on the fly during a negotiation. For example, it could entail working out a seemingly big "cash discount" number and offering it on the spot, knowing full well that it won't take so many points off the margin that the pricing team will throw a fit nor impact their commission such that they care. These are fairly simplistic one or two decimal place fractional arithmetic that are only moderately more complex for the layperson than tallying up the tip at a meal, but it puts a lot of pressure on the customers when they are told, "if I have to walk out of this meeting without closing, I won't ever get the authority to offer that discount again".