Gravel Bike. Yep a catchy marketing term. However they do seem to fit the bill for someone who want's a bike to do almost everything.
You could say :
Road bike............................add fatter tyres and it will be fine on rough and rutted tracks.
Cyclocross bike.................what's wrong with an ordinary bike with fatter tyres with a bit of tread? It'll be fine.
MTB.........................................slap on some slicks and the road is your oyster, it'll be fine.
Downhill................................use your MTB it'll be fine.
Time trial bike..................just lean down a bit and it'll be fine.
Tourer..................................bolt on a rack to your racer and it'll be fine.
Exept of course it won't. Each would do the job but it would be lacking so much.
I don't race, so why do I need a really fast road bike but I do like to go as fast as I personally can, I don't climb mountains or go cross country but I would sometimes like to. I. so far haven't toured, but I expect and hope to be doing so in the not to distant future.
I might conceivably like to ride fast on a touring holiday that would take me off road .
Maybe I am just coming up with arguments for convincing myself that buying that n+1 is in fact essential and being a 'do-it -all' bike it is the most essential bike of all.