I used to use a program called mps-YouTube. It allows to easily play YouTube playlists. Currently it doesn't work anymore, since the Limit has been reached (presumably because every mps-YouTube user shared the same API key).
This made me wonder. You need to register an API key with google, yet you can just use YouTube for free (even adblocking works without any issues).
Since I have to use the API anyway for watching YouTube videos with my browser and don't have an API key, why does mps-YouTube need an API key?
And what would prevent someone from just snatching the key from an open source project like mps-YouTube and using it anywhere else?