I still use my TI-83+ pretty often.

All the years of using it with physics homework and statistics homework and whatnot has wired its usage into my brain.

Sure, I could use R or Python, but when all I need to do is some simple math the calculator is always faster. It has tactile buttons I can operate with one hand, a clear readable screen, and a battery that lasts long. And it's simple.

If mine broke I'd buy another one.