I agree 100% with you that the GMs know who is eligible. But I am going to pose a 100% hypothetical.
What if a GM is thinking of selecting Aubrey. But Aubrey, hypothetically, would rather sit out this WNBA season in order to rehab her sore knee to prevent a long-term situation. She was rested at the end of this season for a reason.
I believe the GM, and league, would like to know. Not sure if teams would draft a second or third round pick that wasn't immediately available.
I repeat, this is hypothetical and is totally my thinking alone and not rumor based.