Because turning the inner knob down from 12:00 doesn't increase input headroom. The input circuitry will clip at the same signal level with it set at 9:00, 12:00, or 3:00. It only affects the signal level after the signal is digitized.
Here's a quick test to help illustrate things:
Start with both inner and outer knobs at 12:00. Increase input sensitivity (outer knob) until the meters show peaking when you clap loudly. You should see both the bar-meter peaking and illuminating the 'peak' rectangle on the right of the meter, AND the input channel indicator flipping its illumination from dark background with illuminated lettering to illuminated background with dark lettering indicating 'analog input stage clipping'. If you keep the input sensitivity (outerknob) in the same position and adjust level (innerknob) while clapping at the same loudness level as before, you will see a resulting change in the position of the bar meter, but the input channel indicator will still invert as each clap peaks in input circuitry. Turn the inner knob down and the analog stage will clip before the digital stage. Turn it up and the bar meter (representing the digitized signal) will clip before the analog stage.
I can't imagine any reason to ever turn level (inner knob) below 12. As for positions above 12:00 and Oade's recommendation, it's speculation on my part, but I suppose the noise of the analog input stage may be greater at higher input sensitivity settings relative to the level of signal which could be apparent when recording very quiet sources which need a lot of gain. If that is the case, then using 'just enough' analog gain to get the signal high enough above noise floor, then adding digital gain with the inner knob (which is the same thing as amplifying digitally on the computer later), may result in a higher signal-to-noise ratio. It is something that a competent tech with measurement gear like Doug Oade would discover when checking out the recorder’s true specifications. But besides being speculative on my part, that assumes a scenario which isn't common with concert recording and also complicates things by making the meters harder to understand clearly. IME keeping the inner knobs at 12:00 except for accommodating minor gain imbalances between channels things is both simpler and without negative consequences.
I am using an Oade Concert Modd’ed R-44, which supposedly has lower analog input stage noise than a stock model, so maybe I’d hear a difference if I was using the stock version and less sensitive mics such as dynamics, but I’ve never noticed problems with excessive input stage noise, even with relatively quiet sources and typical condenser taper mics.
Does that help?