When sending payments, Dutch bank Bunq has basic calculator functionality which allows simple calculations to be performed.
Here’s a video of the calculator in action:
I think an inbuilt calculator would be really useful for sending payments and calculating Bill Splits since it would reduce the need to switch between the Monzo app and a calculator app when figuring out values.
NB: to save space when using the calculator in Bill Splits, the calculation could appear in a bar just above the operator symbols instead of above the result of the calculation.
If you mean switching between apps, having a built-in calculator would make the process of calculating payments or splits much easier and be a nicer UX.