They basically let you write, using a declarative language, what state the server should be in. The software figures out how to set that up for you.

When you want to change the state, you edit the file - say, to add a package.

At any point you can set up an entirely new server from scratch using that same code.

You can't do that easily with bash scripts.

(you could if they worked like virtualenv requirements files but that only works for python's virtualenv...)