The Safety Net That Has to Survive the Move
When you migrate from one platform to another, your organic traffic dips while Google works out where you've gone. Paid traffic is the safety net. If nobody plans for it, the net breaks at the moment you need it most.
"Most platform migrations I've seen treat paid traffic as an afterthought. It isn't. It's the only thing keeping the lights on while Google works out where you've moved to."