if I need to calculate something I'll pull up a console and go into a repl and just ask php or ruby to calculate it for me
I'm guessing you don't spend 100% of your time in front of a computer, and even if you do, in the time it takes you to do that, someone who can actually do mental arithmetic will have approximately calculated that problem and several more, because they can stay in the "flow" of thinking about the bigger problem. As someone who can, it's astounding how many developers out there can't --- pulling out the calculator for computations as simple as 12+51 or 6x128.
It reminds me of the "developers don't need to learn how to type quickly" argument --- yes, you can be productive without, but you'll quickly find yourself to be in a handicap in contrast to others who can write, rewrite, and mentally estimate and compute the results in their head several times faster, saving much time in compiling/running/debugging/etc.
Being able to perform mental arithmetic even approximately is a very helpful skill - buy 6 items at £12.80 and get charged £110? Easily catchable with a quick "6x10+25%" mental approximation.
(I'm fairly sure there's a famous book about this kind of thing but for some reason my brain is stuck on "How To Solve It" by Polya which I don't think is correct.)
I'm guessing you don't spend 100% of your time in front of a computer, and even if you do, in the time it takes you to do that, someone who can actually do mental arithmetic will have approximately calculated that problem and several more, because they can stay in the "flow" of thinking about the bigger problem. As someone who can, it's astounding how many developers out there can't --- pulling out the calculator for computations as simple as 12+51 or 6x128.
It reminds me of the "developers don't need to learn how to type quickly" argument --- yes, you can be productive without, but you'll quickly find yourself to be in a handicap in contrast to others who can write, rewrite, and mentally estimate and compute the results in their head several times faster, saving much time in compiling/running/debugging/etc.