When you are dead drifting with a spinning rod, you have the bail open, and line is being pulled out off the reel. The line is just dumping off with slack though. If your bobber goes down, you have to engage the bail, reel in your slack, and then set the hook.
With the centerpin rod, the line is being pulled off the reel with almost no resistance (assuming you keep it well maintained). This allows you to keep the line almost tight to the bobber while not inducing drag. When the bobber goes down, you just stop the spool from spinning and set the hook. You don't need to engage anything on the reel, and you don't need to reel in slack.
Think about the advantages of bobber fishing with a bait caster vs a spinning reel. I would say a centerpin reel is like a baitcaster in terms of advantages. The spinning reel is easier to use and cheaper though.