By purchasing the game in its beta version:

• you support the work accomplished
• you, of course, acquire the game in its upcoming full version.
• you also gain the opportunity to discover more content than what is currently offered in the free demo up until its full version.
• and, of course, free to provide feedback of your experience

Overall, the goal is to offer those who wish to do so the opportunity to test the game more thoroughly in its current state.