Remember how we all used to say, "But Miss Mathteacher, we can just use a calculator?" and she would put us in our places by saying, "You won't always have one with you when you're out and about, will you?" Then along came the smartphone, with its dozens of apps, the least of which can calculate far more with a few taps than even Stephen Hawking could do in his head. Oh, how we laughed at Miss Mathteacher! What a fool she was, to be overtaken by technology - and so quickly!
The truth, though, is that she's still right. Being able to crunch numbers in your head is still important. It's partly that you should be able to detect when numbers just feel off; for example, when a bill seems too high or when the expenses on your spreadsheet are adding up funny. It helps you to spot anomalies like an extra item where it doesn't belong or a misplaced bracket or equals sign that's causing the whole spreadsheet to go kerblooey. It's also just plain handy for when you genuinely don't have a calculator; after all, sometimes your phone dies. More importantly, being able to run figures in your head is a baseline life skill, like being able to cook. So many other skills rest on it that letting it atrophy is actually dangerous for your ability to think clearly, and not just about math; it's also about your ability to deduce a conclusion rationally from a premise. It makes you harder to trick, and just plain smarter.
That's why, doubtless, you've kept that part of your brain spick and span ... and now's your chance to prove it!