I work in a scientific profession that would require trig functions, etc. My mentor back in the day told me to get the 12c so I did. I couldn’t use it to find the arcsin of anything but I could amitorize all of their loans in the office.
I got the 12c in my MBA program in 1979 or 1980. I was an energy engineer and had the 15c for that work.
Then along came VisiCalc on my trusty Apple ][ and that was that.
But I still use my 15c emulator on my iPhone almost daily! It is superb. And I still remember RPN. After using an early Commodore calculator with prefix notation, it took a while to Grok infix notation, but I came to love it and the very sensible stack.