[codedokode]

I also noticed that digit keys on keyboard in desktop calculators are useless - why would you type in numbers by clicking? It's very slow.

[saila]

They're useful to some people for accessibility reasons.

[fhrow4484]

As well as touch screens!

Accessibility-wise, it gives user choice between on-screen keyboard and the app native one

These days many devices can be used in touchscreen only mode (Microsoft surface etc), so keeping the digits makes even more sense there.