I can't read the mind of the people who write the procedures, but here's a possible line of thought.
In the NG this doesn't happen; you just open the Bus Tfr. How then can you determine which Bus is producing the smoke and isolate it? Help & understanding will be appreciated.
Back in the 'old days' it was assumed that manual intervention would be necessary to clear a large set of fault types. Particularly if you have a flight engineer available to go through a longer list of procedures while the capt/f.o. fly the plane. But now a goal of systems design is to reduce crew workload by the use of 'smarter' systems*. If the fault progresses to a state of imminent danger, automated protection should be able to clear it. The 'open Bus transfer' step is a quick procedure to ensure that a remaining good generator will not inadvertently close into the faulted bad side should the automated systems not be smart enough.
*Why the designers of the 777/787 power systems saw fit to eliminate a type of automatic bus/breaker protection which has proven itself over time and what they think they've replaced it with, I can't say. I wasn't there when that paper was blank.