You are wrong, normal people, not geeks like us, can't even figure out 2% of something. It sounds crazy but its true.
Actually, I know a lot of people who don't know that half a liter and 50cl are the same thing - and I live in a metric country.
Math literacy is very very low nowadays. People from the previous generations where pretty good (my mother always checks by hand that the calculator is right
So, yes, a calculator on a watch, or anything actually, is useful for a majority of people. I guess a calculator inside the brain will be the ultimate thing....
Moreover, not all the operations are that easy. For instance, when I'm on holidays with friends, I use Tricount a lot. Because it can track operations we do all the time, like I bought the groceries but you paid for the restaurants and Bob paid the tickets, so, who owns who what ? Tricount would be very useful on a watch, because the phone is usually in the backpack.
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The apps don't use the processor of the watch instead they bluetooth between the watch to the phone to calculate.
The WWDC is coming soon, hopefully we should have good news on that front...