Amazing ! I never realised Arrau used one.
I'm afraid it's just a physical thing with me - a perfection of the physical yoga (yes, that's the right word - yoga) of piano playing.
As with most worthwhile studies in life, there are regions of commonality and regions intrinsic to the individual. Knowing which is which must be very hard for a teacher to discern.
(I am not that crazy about metronomes, and I think most of the time people misuse them, as in the misguided practice procedure of trying to speed up a piece by starting slowly with the metronome and going up a notch every repeat).