When you sign-up you get two API keys, one for your productive deployment, the other one for development and testing.
This way you can cleanly separate different implementation purposes, e.g. if you want to test a new release, try a different configuration or deploy a new feature to a beta group?
Because you get a lifetime free package of 500 concurrent devices licenses with the second API key, it also does not eat up your acquired production licenses. So double win for you.
If you signed up before February 2017, you only got one key. If you also want to have another one, just let us know at firstname.lastname@example.org and we'll create one for you.