I'm an accountant old enough to remember the days when every accountant had one on their desk. My first firm had plenty of old timers clacking away on these things all day. I actually find that super satisfying, real soothing in an odd sort of way.

I had one at my desk when I was a rookie but being younger I used excel as god intended. A partner came over to my desk once to go over my work and he ran some numbers on the ten key I never used. I said no that's wrong, the real number is such and such, and couldn't figure out how to use the damn thing. The partner found it hysterical that a licensed cpa with degrees and everything didn't know how to use a ten key. (I find it amusing and depressing when they don't know how to use excel, but who asked me.)

Accounting is a very conservative profession slow to change, so a lot of old school partners at firms still have those big old things on their desk. I worked with a dinosaur for a while who knew exactly how old and out of touch he was and didn't apologize for it. Guy would print things out on paper and bang out a bunch of numbers on his ten key that was older than me. He knew his shit though and was really darn good in his own way, just..really really old..but yes you are right there's no reason except nostalgia or habit.