I'm sure others here know much more about it, but my understanding is the NFB loads originated in order to take advantage of the new powders in existing BP proofed guns. While the new full nitro loads were vastly more powerful, there was obviously a huge market with older guns. Smokeless allowed the same ballistics at lower pressure, or as applied in NFB loads, better ballistics (usually a heavier bullet) while staying within the the BP proof.

During that transition period, some guns were then actually made speciifcally for NFB loads. This was probably because they could be lighter and could use existing designs and could utilize those stockpiled actions that were on hand. They still offered an improvement over BP loads while the marketers of the day could trumpet them as 'new'; after all, who'd want a BP gun when cordite was available?

They also got the job done for those not needing maximum power. Off of the top of my head I believe the 500 BPE uses a 340-360 gn bullet, the 500 NFB a 440gn, and the 500 nitro a 570 grainer at a higher velocity. A sort of intermediate load, but much closer to the BP than the nitro end of the spectrum.

Bob