That's a good question.
imho Octopus doesn't do anything that Ansible can't do (they're both great at executing stuff on remote nodes), but Octopus is very geared towards deploying nuget-packaged artifacts, while with Ansible you'd have to write a good portion of that yourself.
I don't think there's a native module for web deploy (yet), but it should be fairly easy to write one (at least a simple one)