We all know the term was invented by the industry to sell bikes, but the need is legitimate. Mountain bikes have gotten so specialized and so good over the last couple of decades that they are now on trails previously not thought possible on a bike. This has opened up many thousands of miles of milder terrain between the road bike and the mountain bike. Thus, the "gravel bike". The term is actually pretty good because "gravel" implies off-road, but not incredibly steep or uneven. The surface you ride it on doesn't actually have to be gravel.
Edit: Kind of like the "Bridge ices before road" signs replacing "Bridge may be icy", a gravel bike is trying to explain something a little complex in just one simple term. The term by itself doesn't totally make sense.