With my HP11C I can find out in under 5 seconds where something might resonate. With a PC I can only do that if I have the proper formula entered into a spreadsheet. Which also needs to be open, no time to wait for some program to load.
That's pretty useless when you are at a client in the lab, the table is full of stuff and the next computer is over yonder in a corner, with someone whom you don't know sitting in front of it. "Hey, dude, could you let me have your PC for a minute?"