Ian, this is what Peter Gannon said when he was asked this a few weeks ago:

The idea is that the same thing would happen if you handed a real musician that lead sheet. If you were playing with Herbie Hancock as your live piano player, you wouldn't be surprised when "lots of 9ths & 13ths suddenly appear" in Herbie's playing, that's considered normal (or natural as we call it).

Similarly, if you gave Herbie this leadsheet...

(#1) | Gm7 Gm9 | C13 c7b9 | FMaj13 F6 | F69 |
.... he's likely going to look at it, and think "

(#2) | Gm7 | C7 | FMaj7 | |

... that's what the natural arrangement feature does. So you hear nice phrasing and smooth voice leading when the player is freed up to play something natural like the simple II-V 1 (#2)

If you told Herbie "no, you must play the chords exactly as written in #1" - he'd do it, but it's likely that what he played wouldn't sound as good as if you let him play #2 (because then he could think of good phrases to play. This is what Band-in-a-Box 2017 does. (of course you can configure this option to be on/off for songs/tracks/globally etc. It defaults to ON for Jazz styles. You can turn it off in Prefs-Arrangments globally, or for the song only in Song-Settings.
It works for RealTracks and MIDI SuperTracks.

BIAB 2024 Win Audiophile. Software: Studio One 6.5 Pro, Swam horns, Acoustica-7, Notion 6; Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus Studio 192, Presonus Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors