Transient behavior as in time-varying behavior, such as what you would find in a bubble column or moderate Re flow past an obstacle in a channel etc. We know that these flow configurations exhibit time-varying behavior physically. For instance the periodic vortex shedding in the latter case etc. So if you want to accurately capture such phenomena, you stand a better chance if you converge in every time step. If all you are interested in is steady state solution, then you can safely march faster in time.