To do math without being in front of a computer.
I find calculators to be easier to use than a computer screen.
I write a PowerBasic program to do serious math.
I had a fairly horrible programmable-gain-amplifier design problem yesterday, the issue being to pick in-stock+available 0.05% resistors in a string to get the required gain settings within a software calibration headroom tolerance. I hacked the ideal values with a little algebra and used LT Spice DC analysis to evaluate the gains and CMRR with available resistor values.
John