I was always under the impression typical BF 3 was hydroscopic and the reason being it prevented any water entering the system, say from caliper immersion, mitigating that water because it would be dissipated throughout the system rather than being concentrated/stuck in one spot, leading eventually to rust/corrosion. Of course, that hydroscopic aspect would gradually lower the fluid boiling point, and eventually once enough water was absorbed rust would soon follow. The newer 5 Fluids resist water absorption, but that does not prevent the downsides if when it happens but maybe speed up the rust process.