A transitional architecture is a useful practice used when replacing legacy systems. Much like scaffolding might be built, reconfigured and finally removed during construction or renovation of a building, you often need interim architectural steps during legacy displacement. Transitional architectures will be removed or replaced later on, but they're not just throwaway work given the important role they play in reducing risk and allowing a difficult problem to be broken into smaller steps. Thus they help with avoiding the trap of defaulting to a "big bang" legacy replacement approach, because you cannot make smaller interim steps line up with a final architectural vision. Care is needed to make sure the architectural "scaffolding" is eventually removed, lest it just become technical debt later on.