The advantage to HTTP validation is that it's simple. No messing with DNS or API keys. Just fire up your server software and tell it what your hostname is and everything else happens in the background automagically.
And this is different from DNS how exactly? The key and resulting cert still needs to be distributed among your servers no matter which method is used.
With dns-01, multiple servers could, independently of each other, fetch a certificate for the same set of hostnames. Not sure if it’s a good idea though.
I guess it depends on the CA, but some do. Let’s Encrypt does, for example. I guess it’s useful for HA deployments, where load balancers might be spread out across multiple datacenters and stuff like that.
Not really, just forward .well-known/acme-challenge/* requests to a single server or otherwise make sure that the challenge responses are served from all instances.
