Some applications require direct hardware access and only run in OS 9, lot's of scanners would fall into this.
Of course there are some other apps and hardware that benefit from OS 9s more direct hardware access, or only have OS 9 drivers.
Unfortunately the apps that need direct hardware access, and/or only have OS 9 driver may require someone to boot into OS 9 to use them.
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But the classic Mac OS has almost always required ROM updates every time a new machine was released, along with new apps/utilities, extensions, control panels, etc.
Since Apple froze OS 9, and stopped the majority (nearly all) ROM file updates for the Mac the hardware couldn't really change much.
Which is why the core Mac has stayed static for so long.
When Apple released new machine PowerMac MDD Rev B, iMac 17, PB 12/17 with the new chipsets -- the OS 9 ROM file was broken utterly and could no longer boot into OS 9.
Sure somebody could patch the ROM file and the new Macs bootROM to get the machines to boot into OS 9, it's been done before, but nobody has done it this time.