Main reason for using an interrupt is the learning experience. See what is does, how it behaves.
Another was "keeping the main loop tidy".
Most of my code has been used for the mode where I use two PIR sensors to detect someone walking up or down the stairs. That will be one of the selectable modes. I still need to figure out how to put this in a separate function.
Before I rework this modeselect into an "in loop" dectection: is there a reason why using an interrupt for this is 'not done'?
Thank you for the assistance both dougp and UKHeliBob