you assume the calculator produces the correct result every time

The calculator itself sure. But the whole process of calculating using calculator? Definitely not 100% always correct. You can miss the buttons, hit the entirely wrong ones, you may put in entirely wrong thing. So if you care about the result you double check, especially if you get something unreasonable. The calculator being perfect is quantitative difference, not qualitative once. The process would be exactly the same if the calculator made 1/100 operations wrong.