Mainly for back of the napkin calculations to evaluate feasibility of solutions/architectures/algorithms, and to evaluate whether to invest in an opportunity.

Granted, they could all be replaced by computer evaluation, but when someone shows me some numbers and wants a quick opinion, I've found that if I ask if I can take a moment to type it into a device people are more likely to go "that sounds complicated, never mind" but if I only jot down some numbers on paper they are more likely to put weight on the conclusion.