What’s… in aid of?
Definition 1 of 2
Pro Tip 1/3Colloquial Usage
Used in casual conversation, might not be suitable for formal contexts. 
He bought another car? What's that in aid of?
Pro Tip 2/3Expression of Puzzlement
Shows you're confused or perplexed by someone’s actions. 
You're taking a yoga mat to a football match; what's that in aid of?
Pro Tip 3/3Unusual Behavior
Often used when someone is behaving in a way that's unexpected or not understood. 
Why are you wearing that costume to work? What's that in aid of?