Originally Posted by
WhyFi
It's a quick and dirty generalization, but -
road race : road endurance :: CX : gravel
vs CX, gravel tends to have more relaxed geometry (longer wheelbase, longer chainstays, lower BB, etc), more provisions for water bottles, fenders, racks, and larger tire clearance.
As for why you would need one - I don't know. Why would you need any bike?
The tire clearance thing is bike. IIRC, cross rules stipulate a max 35 tire width. (I could have that wrong, it's been a long time since I raced cross.) For gravel, a lot of people run 40+ tires. In fact, a lot of riders even run smaller wheels with bigger tires - essentially 26in MTB wheels and tires.