Am I missing something?

Card Text:
As an additional cost to play Guardians Reborn, exhaust any number of Green Lantern characters you control with total willpower 3 or greater.

Recover target stunned Green Lantern character you control.

It looks to me like there is no additional cost to play GR, unless you (for whatever reason), want to exhaust your characters. I mean, "any number" can be zero, right? I must be missing something.

Is there a reason you would actually want your GL characters with 3+ willpower to be exhausted?

You have to exhaust what! It's a cost for you to pay, you MUST pay if you want to use it, it is not an effect that you may oblige. There is no way (at the present moment) to be able to exhaust 0 characters and have 3 willpowers to use its effect.

Nevermind, I misunderstood the wording on the card. I thought it was asking you to exhaust "any number" of characters that individually had 3 or greater willpower, not any number of characters whose willpower adds up to 3 or greater.