My understanding is that the bacteria generally lives on the surfaces within the aquarium. When you dump the BioSpira in the bacteria is floating in the water, and needs a chance to settle into it's new home. Were you to do a WC, especially a large one, before that happened you'd change out all the bacteria you just added. Once the bacteria is establised, it should multiply where it's at, so once it's had a chance to get on the surfaces it will multiply there as well.
You are correct, but I was referencing the fact that certain conditioners can kill off the beneficial bacteria as H3D stated, not the WCs themselves (the conditioner added to the new water).