"beta" is usually full game with some unfinished/unbalanced parts. "demo" is playable, but in some way limited version of full game, for example one level or two. It's normal to publish mods as alpha/beta or as demo and there are many reason for that: to reserve theme or name (see eye of the beholder mod), to get feedback, to test some solutions you have (usually there is not a team of developers behind mod and feedback from players is only way to imprve your work and publishing is only way how to bring your mod to them). Check other mods and you will find that they usually have many versions, and many of them are published in the same way and you can follow their development proces (from level one to ten, etc). Also many of them are published just to show something to others and they are not "games" - Cows on Wheels for example...