Mathematics is about reasoning. Calculations and arithmetics are merely a little insiginificant part of it. I believe mental math should be encouraged at early stages of education, as it develops cognitive skills, memory and brain plasticity; all research confirm this. Sure, calculating 65*82 is tricky to do in head, but if you understand that this is equivalent to (60+5)(80+2) and work from that then it suddenly becomes approachable for everyone, you just have to reason this out in your mind. My algebra teacher once said something which perhaps translates poorly but let me try to convey what he meant: "A mediocre mathematician seeks analogies between problems, so that they can solve new problems using tools they are already familiar with. However a good mathematician seeks analogies between analogies". Will you ever require mental math? Probably no, but consider it a workout for your brain, which creates neuroconnections which will later come in helpful when learning new stuff and needing to understand new, complex concepts quickly