A lot of devops tools, including Ansible and Salt, have higher-level concepts of ordering across hosts. In some cases this is implicit in the way the tool operates and people just take advantage of it, but in others (like Salt's orchestrate runner) the ordering of operations across hosts is explicitly defined by users. That can be useful for scripting blue/green deployment cutovers, pushing out new load balancer rules, etc.

> I'm actually curious about things like ansible; what exactly does it do that you couldn't just do with a bunch of redo scripts?

That might be more of a philosophical question :-) Most of those tools could be replaced with a sufficiently intelligent set of scripts. I think they get their popularity from providing more convenient syntax, building in parallelism for applying changes across hosts, offering cross-platform ways of doing common admin actions like creating accounts, etc.

But if you look at them the right way, they definitely share a lot in common with build systems, particularly when it comes to managing dependent actions.