My understanding is that on bright sunny days in clearer condition, steelhead will hide deeper than usual. You won't find too many at the shallow edges (except at first light). Float fishing may not reach some of these deep sitting steelies. Of course, you can try BBing on cloudy days, but steelies are more spread out to shallower areas, which float fishing will do just fine or even better.
Another situation of BBing is when steelies hide underneath strong top flows, whereas the bottom flow is relatively calm. Take the Osborne Pool for example, the top end of the pool may hide steelies underneath strong foamy flow at the surface from the rapids above. Fishing a float there can be too fast as the strong surface current pushes the floats down. BBing will take care of this problem, so you can fish deep, and slow down the presentation.
Just my humble 2 cents on the subject of BBing for steelies. Others may have their own experiences.