The 'where
each user is' is implicit, the expectation is that you're some kind of global SaaS, and you want low latency where
ever your users are.
Sure you can do that with any cloud (or multiple) that has datacenters in a suitable spread of regions, but I suppose the point (or claimed point, selling point, if you like) is that that's more difficult or more expensive to coordinate. Fly says 'give us one container spec and tell us in which regions to run it', not 'we give you machines/VMs in which regions you want, figure it out'. It's an abstraction on top of 'battle tested proven bedrock' providers in a sense, except that I believe they run their own metal (to keep costs down, presumably).