Image source: KFC 225 Pilot's Guide (PDF)
In a dual-axis AP, when no lateral mode is selected, a wing leveler (ROL) mode is activated and can't be disengaged unless another lateral mode is selected. Similarly for pitch attitude hold (PIT). (Those two modes are shown above.)
But why, for example, can't the AP hold only the altitude, and the pilot steers laterally. As far as I know any AP with more than one axis doesn't allow single-axis operation. Control wheel steering (CWS / attitude hold) is not what I mean.
Why's that? Regulation? Design limitation?
One benefit of allowing single-axis – I think – is for climbs and descents when there is no speed hold mode. The pilot lets the plane steer left and right, and they control the climb/descent by manipulating the power/trim. Instead of guessing the proper vertical speed – which keeps changing if it's a long climb/descent, unlike trimming for airspeed.