[roondar]

This is a pretty common style to use on the 68000. It makes sense to use it over a direct address because it's faster (16 vs 20 cycles).

Edit: that was a bit short, so let me add a bit more about why I think this makes sense.
To me, this method makes a lot of sense for another reason: using these offsets means you're closer to how the designers/chips looks at the registers. The $dff000 is just a prefix for the 68000, in actuality the registers are numbered from $000 onwards. You can see this in the HRM where you'll notice that the register tables they give for never include the $dff000, just the part after the $dff. It also fits better with the Copper, which omits the $dff as well.