My mountain bike sucks for gravel. Fat tires roll slow.
My race bike doesn't do loose surfaces. Skinny tires ride, harsh & sink in.
If you can fit at least 32-35mm tires wide on your road bike & don't actually ride gravel, then a dedicated bike is redundant. Conversly, if you do ride logging/forest roads & can't fit the tires, then the new (to you) bike might be a smart investment.
It depends on where you ride too. Some roads are better than others.