A campaign earns once, on the day you press send. A flow earns every day, because it fires on something the customer did rather than something you remembered to do.
It also arrives at the right moment. Nobody is more likely to buy a product than the person who put it in a basket an hour ago, and no email you write in advance can be that well timed.
Once built, a flow needs checking and improving, not rewriting. That is why I price the build and the monthly management separately on the pricing page.