"Powers?" Follow-up question

[Interference]

I was playing a game with a friend and I used Weavile SW's Dark Engage PokePower even though it wouldn't have changed any damage (no special darks, defending didn't have a weakness to dark). My friend promptly Power Sprayed it, which is what I was hoping he'd do. I moved on with my turn when he noticed that using the power would've changed nothing and was pretty irritated.
Using the Dark Engage didn't change the game state that particular time, other than when my friend wasted his Power Spray to stop it, so I'm a bit confused if this is allowed.

[PokePop]

Yes. That was allowed.

But besides that, you're confusing "change to the game state" with "worthwhile change to the game state".
You would have changed a Pokemon's type to Dark. That would be a change to the gamestate.
The fact that it wouldn't have helped you doesn't make it not a change.

In any event, even if your active were already Dark type, you could still have used the Power.