When written correctly, it's idempotent. I've done a lot of server management with bash and it's a lot easier to achieve idempotency with something like Chef.

It's usually more concise because idempotency comes by default. Instead of saying "create this file" (which might throw a 'file already exists' error the 2nd time you run the setup script), you say "ensure this file exists".

Some bash commands are idempotent too (e.g. apt-get install), but it's not something you can rely upon, and you often have to code the idempotency in yourself.

What if you have to upgrade a software package and add a new config file that is different on every server. I guess you can do that via a horrible sed command, but having native template support with variables is pretty nice.

Same for things like "tune the amount of worker processes depending on the amount of CPU cores the machine in question has".

Things like Ansible/Salt and similar tools cut out a lot of the boilerplate. There are also plenty of modules you can reuse without having to roll your own. You can achieve the same results using Bash/Perl/Python but a lot more effort is needed.