The B-stance deadlift provides a middle ground between unilateral exercises and bilateral exercises. The staggered stance of the B-stance means that the working leg can be challenged further than with a bilateral RDL, as the supporting leg does not work as hard, but is less challenging than a single leg deadlift as the supporting leg helps with balance.
It’s a great regression for those who want to work up to a single leg deadlift. The B-stance can be loaded heavier than the single leg variation.