Protects against certain kinds of transient hardware faults that are common enough to worry about in safety-critical systems, I think.
But if there's a human in the loop and a manual alternate control pathway, detecting a disagreement allows you to cue the manual operator and transfer control to them. Or fall back to a much simpler system of computer aid.
With 1, hardware failures are extremely hard to detect at all, as even your computational checks for internal consistency are subject to mutation.
Unless all 3 different give different results, two failures and one correct.
IIRC the shuttle had a 3+1 system 3 as a cohort with voting and if they couldn't reach consensus the 1 was a minimal system that could keep the lights on.