The past demo of KSP was 0.13, it seems a life time ago as sooooo much has changed. 0.13 is almost a different game.
So the demo has been updated to almost the current build. The current build being 18.4. while paying fans of KSP will continue to update this demo wont. However you are getting a great play with a very filled version.
You can build Spaceplane, Space ship, Space stations, Space bases and even construct a massive ship in orbit with plans to send it out to SPACE! Not many survive as they wobble themselves to death leaving a massive ring of debris....maybe you can get the right ship to do it?
I will leave these videos for people to be convinced to at-least play the demo.
And of course the flight clubs Quick look here on Giant bomb.