Salt and Puppet are useful for managing a fleet of servers running various applications, especially when you need to scale those applications horizontally or want geo-distribution.

Ansible can also do that, on top of literally anything else you could want - network configuration, infrastructure automation, deployment pipelines, migrations, anything. As always, that flexibility can be a blessing or a curse, but I think Ansible manages it well because it's so KISS.

RedHat's commercial Ansible Automation Platform gives you more power for when you need it, but you don't need it starting out.

A combination of HashiCorp Packer and Ansible means I can "publish" a VM ready-to-rock image to a public cloud provider gallery and use it to run a VM in said cloud.

Ansible-Lockdown is another excellent example of how Ansible can be used to harden servers via automation.

The other commenter already answered the usecase question, for self-hosting you will likely find ansible the easiest entrypoint.

It is in general the simplest of these systems to get started with and you should be able to incrementally adopt it. There is also a plethora of free online resources available for it.