I think the strategy is pretty obvious: Cannondale is giving its customers options and allows them to use their Synapse not just as a “traditional” endurance road bike, but also a gravel bike. And even offroad, the Synapse with knobby(ier) tires still has the geometry, and thus, the feel of a proper road bike. I reckon this will be very popular. Plus, for customers who don’t care as much about speed and want extra comfort, you can get 40 mm slicks these days. This is very much the philosophy of one of the first modern gravel bikes, Open’s UP.
And what about customers who don’t want to take it offroad? Well, just stick to the tire width you want and don’t use the extra clearance.
How do you know? Where I live, gravel bikes have taken over from hardtails as the default bike for people who want to get around. That’s because they are very versatile, light, fast and robust while being comfortable due to large tires.
I can see myself putting 45 mm slicks or so on my commuter. The extra width would allow me to ride some gravel, give me extra traction for when I pull a trailer (important for breaking performance) while still being quite fast. But it need not be a setup optimized for all-out speed.
There are a few assumptions baked in here: the first is that it will be slower due to extra tire clearance. Are you sure about that? There are some very aero bikes that have wide fork legs to be aero. I reckon there are many gravel bikes that are faster than many road bikes.
Secondly, the added flexibility will be a boon to most customers. Once they realize that they can turn their new road bike, which rides like a proper road bike by simply changing tires, they’ll be happy.
Thirdly, it might simplify manufacturers’ line-ups (Cannondale isn’t the only company doing this, e. g. BMC’s new Roadmachine very much follows the same recipe): You have your traditional race bikes (which these days also have up to 33–35 mm tire clearance!), endurance/groad bikes and then chunky gravel bikes with clearance for MTB tires.