I like to have a calculator on my desk as a kind of ‘burst capacity’ for when I’m busy keeping an important mental process running. It’s like having a dedicated chip (with its own private chunk of RAM) optimised for simple arithmetic operations, when such a need arises mid-flow, always available to use without pausing to reallocate any memory elsewhere when everything is running at capacity trying to make some complex decision. And by ‘memory’ here, I mean the combined scratchpad made up of my brain’s thought loop and the windows arranged on my desktop. If I’m at capacity, just changing the focus of windows on the desktop (eg. to bring up a software calculator overlay) can break my train of thought, meaning the spinning plates start to wobble and fall down, and then I have to start over on the wider task, all because I got stuck on something silly like 14*2.5. With a pocket calculator on my desk, I can safely turn my head to it for a moment, do the sum with almost zero effort, and turn back to the monitor, and the plates are all still spinning.