Is there any difference between an API key
and an application key?
They provide the same level of access.
Application keys are useful if you want to grant access to a specific app, because you can revoke or update just that one key if necessary. If you use your API key then it will be the same for all applications. If you want to change/revoke access them it will affect all apps.
More infos here:
I should have been more clear in my question. I totally get what the advantages of application keys are. What I don't get is what the advantages of the API key are.
In past versions of OctoPrint, there was only one global API key. Newer versions added the application keys but kept the global key for backwards compatibility.