It's still a bit early for the mechanic to be fully fleshed, but the idea is to collect it in various ways and then to use it to slow down a fetish progression.
When a fetish goes too high, you will end up in a kind of sandbox game over. I see you have Domina in your banner, you can see it as Domina with the sandbox ending when you choose to submit, except instead of having 1 sandbox per domina throughout 13 domina, you have 4 or 5 possible sandboxes for each girlfriend throughout 3-4 girlfriends.
To avoid a "game over" and to explore more of the core storyline, you need to collect and use the willpower and resist your girlfriend's domination as much as you can.
Though again, it's too early right now for the mechanic to work as intended. But in time, we'll get there
soulwombat, If you have any difficulties running the game, you can join the Discord and talk with me directly, the link is on the main page. It will be easier to solve the problem if you can send me screenshots and if we can discuss in more detail what is going on.