The point is just to get some traffic somehow so you can check whether anyone wants to play the game you’re about to spend tons of time and money on. Ads are a simple way to drive some traffic. If you can’t get clicks or sign ups, you will save yourself a really bad time by figuring that out before starting to build, and iterating to a better concept/pitch that is validated by real potential users.

And yeah a prototype video sounds like a great way to test people’s enthusiasm.