Slides available here. This repo covers the same concepts, but goes more in depth.
Configuration management (CM) systems allow you to speed up (automation), regulate (version control), and repeat (idempotency*) processes that go into machine setup and configuration.
* = idempotency is a property that asserts that you can run the same set of commands more than once without breaking things.
tl;dr: CM takes you from a world of haphazard development and test environments to a world of quick and easy environment setup.
If you find yourself reusing a single SSH key for different servers, you might get a scary error message like this:
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@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
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IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
If you read a little further, it'll have a line along the lines of...
Offending RSA key in /Users/emily/.ssh/known_hosts:8
If you delete the offending line in your known_hosts file (in this case, delete line 8 in Users/emily/.ssh/known_hosts), the error will go away.
Alternatively, you can run ssh-keygen -R <ip_address>.
In general, DO has excellent up time. But for your own sanity, feel free to check check status.digitalocean.com.